Eppur si muove: The Legend of Galileo
William E. Carroll
If you were to go to Rome and climb the famous Spanish Steps and then head toward the palace to which they lead, you might notice a green marble pillar bearing the following inscription: "The next palace is the Trinità dei Monti, once belonging to the Medici; it was here that Galileo was kept prisoner of the Inquisition when he was on trial for seeing that the Earth moves and the Sun stands still." The first part of the inscription is true, but note the second part: the claim that Galileo was tried by the Inquisition "for seeing that the Earth moves and the Sun stands still." The motion of the Earth and the immobility of the Sun are difficult to see with one's eyes, and one might wonder how Galileo managed to notice phenomena that had escaped the sight of others for centuries. Galileo, himself, never claimed that his telescopic observations proved that the Earth moves. Even today, the Earth's motion seems impossible to observe except perhaps with the eye of the mind.
Although the marble pillar in Rome was erected a little more than a century ago as part of the anti-clerical program of the new Italian government, the impression it creates is pretty much that of most people who reflect on Galileo and his encounter with Catholic orthodoxy in the early 17th century. The controversy between Galileo and the Inquisition, however, is considerably more complicated than either the common impression or the inscription conveys. The legend of Galileo has come to have a life of its own, independent of and, in many ways, divergent from historical reality. Or, perhaps better put, the legend of Galileo possesses its own historical reality.
In the eight lectures which constitute this course on Galileo: Science and Religion, Dr. Hodgson and I invite you to look again at the famous encounter between Galileo and the Inquisition. In today's lecture I will focus on the legend of that encounter. In the following three lectures (two, three, and four) Dr. Hodgson will concentrate on Galileo's contributions to science, and in lectures five, six, and seven I will examine the controversy between Galileo and the Inquisition in some detail. In the eighth lecture I will discuss Galileo as theologian and return, in a way, to the legend of Galileo since an important feature of the legend is Galileo's theological acumen.
Throughout these lectures we will not provide a detailed account of Galileo's life nor a blow-by-blow description of his encounter with the Inquisition. There are many good books which provide such information, and appended to the text of this initial lecture you will find a brief bibliography. Several of the books we recommend have elaborate chronological guides as well as biographical glossaries. We hope that these lectures will enable you to see Galileo the scientist and the theologian in a new light. Our analysis builds throughout the lectures; we often rely on careful readings of specific texts, and thus we expect that you will read these lectures in their text format.
Now let us return to the legend of Galileo. In 1929 the King of Italy inaugurated the Institute and Museum of the History of Science in Florence. The ceremonies coincided with the first national exposition of the history of science, which had been sponsored by the Fascist regime to celebrate the tradition of Italian science. The new museum housed an impressive collection of artifacts belonging to or connected with Galileo. When Mussolini visited the exposition in 1930 he underlined the importance of Galileo as he stood in admiration before the text of Galileo's first astronomical treatise, The Starry Messenger. A contemporary chronicler of this encounter between Mussolini and the textual remains of Galileo observed that this was the first time that the manuscript had for a reader a man of the stature of the one who wrote the text!
The legend of Galileo was well established by the time Mussolini viewed The Starry Messenger. The Grand Dukes of Tuscany had begun to cultivate such a legend soon after Galileo's death in 1642. When Duke Pietro Leopoldo opened the Museum of Physics and Natural Sciences in 1774 he dedicated a special exhibition which commemorated the principal discoveries of Galilean physics. In the 1830's Galileo's scientific memorabilia were moved to a special "Tribuna di Galileo," established in another Florentine palace which was a meeting place for the congress of Italian scientists. Galileo, by now an "Italian" scientist (as distinct from a Florentine or a Tuscan scientist), served an important political role, helping to legitimate the aspirations of those Italian nationalists who longed for the establishment of a single Italian nation.
Today, a walk through the second floor of the Museum of the History of Science reveals the continuing importance of Galileo. Museum attendants stand ready to demonstrate a reproduction of Galileo's device for measuring motion down an inclined plane. The same text which attracted Mussolini's interest more than sixty years ago shares a display with first editions of the Dialogue Concerning the Two Chief World Systems and the Discourses and Demonstrations on Two New Sciences. There is a famous military compass and designs of fortifications Galileo produced for his Venetian patrons, before he became chief mathematician and philosopher at the Medici court in Florence. And, of course, there are the telescopes which Galileo used to discover new astronomical wonders.
The web site of the institute and museum in Florence provides internet access to these exhibits, and a great deal more on Galileo and his times. The internet address of this site is found in the bibliography to this lecture.
Among the telescopes in the exhibit there is a handsomely decorated, oval ivory frame, with a round glass at its center. It looks much like a monstrance used to hold the consecrated host in religious ceremonies such as benediction of the blessed sacrament. The central glass is surrounded by drafting tools and scientific instruments, all etched in ivory. At the top of the inner frame there is a depiction of four moons revolving about Jupiter. The cracked lens at the center of this scientific icon came from the telescope which Galileo used when he first observed these moons. The lens cracked when Galileo dropped it as he prepared to send it to Cosimo de' Medici, after whom he had named these moons the "Medicean stars."
The image of Galileo as patron saint of modern science is further reinforced by an odd reliquary. On a small marble pillar there is a glass sphere which contains the skeleton of the middle finger from Galileo's right hand. In 1737 when Galileo's body was being transported from a small chapel to a great monumental tomb in the Church of Santa Croce, this finger had fallen off the corpse; it has been preserved ever since.
There are few images of the modern world more powerful than that of the humbled Galileo, kneeling before the cardinals of the Holy Roman and Universal Inquisition, being forced to admit that the Earth did not move. The story is a familiar one: that Galileo represents science's fighting to free itself from the clutches of blind faith, biblical literalism, and superstition. The story is as fascinating to many in the late twentieth century as it was to Mussolini, or to nineteenth century scholars and politicians, or to the proponents of the cultural program of the Enlightenment in the eighteenth century. In fact, the famous Galileo codex, which contains most of the documents concerning the Inquisition's treatment of Galileo, was preserved as a result of Napoleon's interest in Galileo. Shortly after the formal annexation of Rome in 1810, Napoleon ordered all of the archives of the Vatican to be transported to Paris; he hoped to create in his capital city a center for the study of European culture. Unfortunately, in the process of shipping more than three thousand wooden chests of documents to Paris (and then several years later, after the defeat of Napoleon, shipping them back to Rome) some of the documents were lost. Three sets of documents in the Vatican archives received special treatment: those concerning the medieval crusading order of the Knights Templar; the 1809 Bull of excommunication of the emperor himself; and the material on Galileo. Napoleon ordered these documents sent to Paris by imperial courier. The "Galileo codex" was itself the result of a collation made by officers of the Inquisition late in the seventeenth century, perhaps to serve as an internal reference. Napoleon's interest in Galileo is attested to by his archivist in Paris who observed that the emperor, having recently been excommunicated, saw himself as a political Galileo who, in ushering in a new order in Europe, was also persecuted by the Church.
In order to pay for the return of the archives to Rome, after the defeat of Napoleon and the restoration of the Bourbon monarchy, the Vatican's representative in Paris had to sell documents to Paris papermakers. It is difficult to know what was lost, but at least two thousand six hundred volumes of documents from the Inquisition were shredded. Despite repeated representations to the new royal government, the French refused to return the Galileo codex, in which Napoleon had taken special interest. Only in the early 1840's did the widow of an exiled government minister discover the codex in her husband's effects in Vienna, and then return it to the ambassador from the Vatican. Napoleon's fascination with Galileo, the result, no doubt, of an already well-established legend of the Italian scientist's difficulties with the Church, served to preserve the very documents which have enabled modern scholars to debunk much of the legend.
The power of the legend was evident in the ways in which the press reported the solemn ceremony in the Vatican in October 1992, when Pope John Paul II appeared before the Pontifical Academy of Sciences to accept the findings of a commission of historical, scientific, and theological inquiry into the treatment of Galileo by the Inquisition in the seventeenth century. In 1981, the Pope established several study groups to reexamine all features of the encounter between Galileo and the Inquisition. In 1984, the Vatican published a collection of original documents related to that encounter, all but two of which had appeared in print before.
In comments before the Pontifical Academy in 1992, the Pope observed that the theologians of the Inquisition who condemned Galileo failed to distinguish properly between particular interpretations of the Bible and questions which in fact pertained to scientific investigation. According to the Pope, one of the unfortunate consequences of the condemnation of Galileo was that it has been used to reinforce the myth of an incompatibility between faith and science: "A tragic mutual incomprehension has been interpreted as the reflection of a fundamental opposition between science and faith. The clarifications furnished by recent historical studies," the Pope noted, "enable us to state that this sad misunderstanding now belongs to the past."
Misunderstandings associated with the story of Galileo were particularly evident, however, in the way the secular press described the October event in the Vatican. The headline on the front page of The New York Times (31 October 1992) was representative: "After 350 Years, Vatican Says Galileo Was Right: It [the Earth] Moves." Other newspapers, radio, and television repeated the same claim. One might forgive journalists the sensational distortion of headlines, but the attendant stories only reinforced the image the headlines evoked.
The story in The New York Times offers an excellent example of the persistence and power of the myth of the Galileo affair. In the guise of a straightforward news account, the author noted that the Pope's address would "rectify one of the Church's most infamous wrongs -- the persecution of the Italian astronomer and physicist for proving the Earth moves about the Sun." But Galileo did not prove that the Earth moves about the Sun. In fact, Galileo and the theologians of the Inquisition accepted the prevailing Aristotelian ideal of scientific demonstration which required that science be sure and certain knowledge in terms of necessary causes, not the conclusions of hypothetical or probabilistic reasoning which today we tend to accept as science.
Galileo had many disputes with his Aristotelian contemporaries, but his criticism of faulty reasoning and hasty conclusions on their part proceeded from a notion of science which he shared with Aristotle. For Galileo, as for Aristotle, science is not first of all an activity, not a program of experimental investigation. Rather, it is that knowledge which is the result of inquiry, expressed with the rigor of syllogistic demonstration. In order to understand Galileo's encounter with the Inquisition, we need to guard against the anachronistic application of contemporary notions of science to events more than three centuries ago. Galileo did not think that his astronomical observations provided sufficient evidence to prove that the Earth moves, although he did think that they called into question the truth of Ptolemaic astronomy. His discoveries of mountains on the Moon and spots on the surface of the Sun challenged the view that the heavens were incorruptible and perfect. That Venus exhibited phases like the Moon showed that planet to revolve about the Sun. In the account of his initial astronomical discoveries, published in The Starry Messenger in 1610, Galileo claimed that his most important discovery was the existence of the four moons of Jupiter, since it removed a major objection to Copernican astronomy. For Copernicus there were two centers of heavenly motion: the Earth, about which the Moon revolved, and the Sun, about which the Earth and the other planets revolved. Opponents of Copernicus thought such a claim absurd. How could the universe be the orderly, harmonious creation it is and have more than one center of motion? As a result of Galileo's observations no one could doubt that there were at least two centers of motion in the heavens: the Earth and Jupiter.
Galileo did hope eventually to argue from the fact of ocean tides, to the double motion of the Earth as their only possible cause. He knew that a truly demonstrative claim in physics had to take such a form, but he was never successful in demonstrating the motion of the Earth. He was working on this argument based on the tides early in the seventeenth century -- he circulated a manuscript privately in Rome in 1615; and the argument appeared in the fourth section of his Dialogue Concerning the Two Chief World Systems, published in 1632. It was the publication of this work which was the immediate occasion for Galileo's appearance before the Inquisition in 1633.
The book, written in the form of a dialogue, compares geocentric and heliocentric astronomies, leaving little doubt as to which ought to be embraced. Convincing as the refutation of Aristotelian and Ptolemaic astronomy might be, the text did not contain a scientific demonstration for the motion of the Earth. The argument from effect to cause, from the ocean tides to the rotation and revolution of the Earth, appeared in the rhetorical form of demonstrative language.
Despite the rhetoric of demonstration, it seems likely that Galileo himself was aware that the argument lacked true probative force. Nevertheless, the book proved to be too convincing a presentation of the new astronomy for the Pope and the Inquisition to accept. An edict went forth from Rome to suspend the publication of the book and Galileo was ordered to appear before the Inquisition in Rome.
The famous trial of Galileo in 1633 needs to be understood in the light of the events seventeen years earlier. As I have already noted, Galileo and the officers of the Inquisition accepted the same Aristotelian notion of what constituted a scientific demonstration. As early as 1615, Cardinal Roberto Bellarmino, Jesuit theologian and the most prominent official of the Inquisition, told Galileo that if there were a true demonstration for the motion of the Earth then theologians would have to abandon the traditional reading of those passages in the Bible which appeared to be contrary. But, in the absence of such a demonstration, and in the midst of the controversies of the Protestant Reformation, the cardinal urged prudence: treat Copernican astronomy simply as a hypothetical model which accounts for the observed phenomena.
If Cardinal Bellarmino had thought that the immobility of the Earth were a matter of faith, he could not admit, as he did, the possibility of a demonstration to the contrary. The theologians of the Inquisition and Galileo adhered to the ancient Catholic principle that, since God is the author of all truth, the truths of science and the truths of revelation cannot contradict one another. In 1616, when the Inquisition ordered Galileo not to hold or to defend Copernican astronomy there was no demonstration for the motion of the Earth. It seemed obvious to the theologians in Rome that the Earth did not move and that the Bible confirmed this fact. The disciplinary decree of the Inquisition was unwise and imprudent: but it was the result of the subordination of the interpretation of certain passages of the Bible to a geocentric cosmology, a cosmology which would eventually be rejected. In lectures five, six, and seven I will examine in some detail these events of 1615 and 1616, for they represent, I think, the key to understanding the "Galileo Affair." Galileo's trial in 1633 concerns his violation of the injunction given to him in 1616.
In the presentation of the findings of the recent papal commission to Pope John Paul II, Cardinal Paul Poupard, head of one of the commission's four working groups, noted that the erroneous conclusions of the Inquisition's theological experts were the result of "a transitional situation in the field of astronomical knowledge and of an exegetical confusion regarding cosmology." The theologians "failed to grasp the profound, non-literal meaning of the Scriptures when they describe the physical structure of the created universe." Galileo's judges, "incapable of dissociating faith from an age-old cosmology, believed quite wrongly that the adoption of the Copernican revolution, in fact not yet definitively proven, was such as to undermine Catholic tradition, and that it was their duty to forbid its being taught."
Within the history of Catholic thought the conclusion of the theologians of the Inquisition was somewhat of an aberration. As early as the fourth century, St. Augustine had warned against using scientific theories to provide definitive interpretations of biblical texts.
Galileo often quoted Augustine when he argued that the Bible ought not to be used to determine the truth or falsity of scientific propositions which are not central to religious belief. Galileo liked to repeat the remark attributed to Cardinal Baronius: the Bible teaches us how to go to heaven, not how the heavens go.
According to the account in The New York Times, Galileo defended himself before the Inquisition in 1633 "by saying that scientific research and the Christian faith were not mutually exclusive and that the study of the natural world would promote understanding and interpretation of the scriptures. But his views were judged 'false and erroneous.'" Galileo, thus, is portrayed by The New York Times as being convicted by the Inquisition for affirming a compatibility between science and the Bible. But, on the contrary, the Inquisition accepted the view that science and faith were complementary. In 1633 Galileo was accused of disobeying the 1616 injunction not to defend Copernican astronomy. The Inquisition's injunction, however ill-advised, only makes sense if we recognize that the Inquisition saw no possibility of a conflict between science and religion, both properly understood. Condemned as "vehemently suspect of heresy," Galileo was required to recant, and he was placed under a kind of house arrest at his villa outside Florence until his death in 1642. From beginning to end, the official actions of the Inquisition were disciplinary, not dogmatic, although they were based on the erroneous notion that it was heretical to claim that the Earth moves.
The documents Napoleon helped to preserve reveal the decision in 1616 of the committee of theologians who advised the cardinals of the Inquisition. The theologians concluded that the proposition that the Earth moves and the Sun stands still is false, scientifically, and heretical, in that it contradicts certain passages of the Bible which claim the contrary. Although the Inquisition formally admonished Galileo not to hold or to defend Copernican astronomy as a result of the opinion of its expert theologians, the opinion remains just that, an opinion. Conclusions of theologians are not sufficient to constitute Church dogma (not in the seventeenth century, not even in the twentieth century). Official teachings of the Church depend upon the authority not of theologians but of popes and ecumenical councils. Even though Galileo was ordered to act as though it were heretical to hold that the Earth moves, it does not follow that the Church taught that the immobility of the Earth is a matter of faith. In fact, there has never been such a Church teaching.
In a sense, however, the Church does teach in her disciplinary acts, but teaching understood in this looser sense does not in itself constitute the authoritative proclamation of Church dogma. Perhaps officers of the Inquisition in the seventeenth century, or commentators in the twentieth century, fail to make this distinction or trivialize it, but it is a crucial distinction to keep in mind if we are to understand the relationship between the Catholic Church and science. In dealing with Galileo the Inquisition acted by the authority of the pope, but the authority was juridical not magisterial.
In 1616, in addition to the specific injunction given to Galileo by the Inquisition, the Congregation of the Index of Forbidden Books ordered that Copernicus's De revolutionibus be suspended from publication until it could be corrected and that works by two theologians (Paolo Foscarini and Diego de Zuñiga), who interpreted biblical passages on the basis of Copernican astronomy, were to be prohibited altogether. The order of the Index was an official, public decree, quite different from the private and personal admonition given to Galileo. Again, however unwise, the action of the Index remains disciplinary. The prohibition of the Index was gradually relaxed. The 1757 edition of the Catalogue of Forbidden Books did not include books that favored heliocentric astronomy. In 1820, Pope Pius VII sanctioned the granting of the imprimatur to works presenting Copernican astronomy as true and not merely as hypothetical. The failure to change Church discipline more expeditiously contributed to the myth that there was a fundamental conflict between faith and science.
Galileo's formal recantation in 1633, despite the claims of The New York Times, did not "save him from being burned at the stake." The Inquisition's own rules would prevent the use of torture on a man of Galileo's age, and there is no evidence that there was any consideration of his being burned at the stake were he to refuse to recant.
The author of the article in The New York Times was more right than he realized when he observed that "the dispute between the Church and Galileo has long stood as one of history's great emblems of conflict between reason and dogma, science and faith." But what the author took to be an accurate account of the encounter between Galileo and the Inquisition is part of a long-standing legend, far from the truth. Consider the following analysis he offered: "The Vatican's formal acknowledgment of an error . . . is a rarity in an institution built over centuries on the belief that the Church is the final arbiter in matters of faith." But the error which the Church admitted in the case of Galileo is an error of judgment; the Inquisition was wrong to discipline Galileo, but discipline is not dogma.
Although the legend of Galileo's encounter with the Inquisition has its roots even in the seventeenth century, I want to call your attention to developements in the nineteenth. Early in that century, Auguste Comte, one of the founding fathers of the modern social sciences, argued that humanity was laboriously struggling upward toward the reign of science, and the principal opponent in this struggle was a reactionary theological and metaphysical view of the world. For Comte, Galileo represents the modern spirit's freeing itself from the stultifying grasp of theology and metaphysics. Galileo's "odious persecution" will remain forever, according to Comte, an exemplar of the "first direct collision" between modern science and the old view of the world.
The nineteenth century was the great age of positivism, which saw modern science as the pinnacle of human thought. For the positivists, science was objective, inductive, and experimental -- and it was born in the great revolution of the seventeenth century when geniuses such as Galileo and Newton succeeded in overthrowing the heritage of Aristotle. Thus, the Inquisition's treatment of Galileo was but one of the attempts to impede the inevitable progress of the human mind. The legend of Galileo's persecution by the Inquisition had become part of the larger story -- also widely accepted -- of the Scientific Revolution. The more one saw that Revolution in terms of the victory of the modern scientific method, a method, so it was claimed, which Galileo pioneered, the more it was easy to accept what had become the common wisdom of the Inquisition's attempting to thwart scientific progress to protect the literal truth of the Bible.
By the second half of the nineteenth century the condemnation of Galileo had come to be seen in messianic terms. The figure of Galileo took on an almost divine role in the redemption of mankind from the dogmatism of the past. The great conflict between truth and falsehood had several heroes, and Galileo was among such a pantheon.
An important modern source of the notion that the Galileo affair is a central chapter in a long history of warfare between science and religion can be found in the debates in the late nineteenth century over the reception of Darwin's theory of evolution. Increasingly, this metaphor of warfare served as a principle in the modern world's understanding of its own history. The legend of Galileo was important evidence for the purported truth of this interpretation. At the same time the legend was held captive by this interpretation: so much so that, even today when we know how false the legend is, it remains difficult to reject it. This is particularly true in the United States where Andrew Dickson White's History of the Warfare of Science with Theology in Christendom (1896) enshrined what has come to be a historical orthodoxy difficult to dislodge. White used the example of the "persecution" of Galileo by the Inquisition as an ideological tool in his attack on the religious opponents of evolution. Since it was so obvious by the late nineteenth century that Galileo was right, it was useful to see him as the great champion of science against the forces of dogmatic religion. White's account may sound a bit extreme; nevertheless, we should be able to recognize an affinity between it and the persisting legend of Galileo:
[Galileo's] discoveries had clearly taken the Copernican theory out of the list of hypotheses, and had placed it before the world as a truth. Against him, then, the war was long and bitter. The supporters of what was called 'sound learning' declared his discoveries deceptions and his announcements blasphemy. Semi-scientific professors, endeavoring to curry favor with the church, attacked him with sham science; earnest preachers attacked him with perverted scripture; theologians, inquisitors, congregations of cardinals, and at least two popes dealt with him, and, as was supposed, silenced his impious doctrine forever. . . .
The whole struggle to crush Galileo and to save him would be amusing were it not fraught with evil. There were intrigues and counter-intrigues, plots and counter-plots, lying and spying; and in the thickest of this seething, squabbling, screaming mass of priests, bishops, archbishops, and cardinals, appear two popes, Paul V and Urban VIII. It is most suggestive to see in the crisis of the church, at the tomb of the prince of the apostles, on the eve of the greatest errors in church policy the world has known, in all the intrigues and deliberations of these consecrated leaders of the church, no more evidence of the presence of the Holy Spirit than in the caucus of New York politicians. . . . [Vol. 1, pp. 130-1, 136-7]
The debate over papal infallibility, formally defined at the First Vatican Council in 1870, as well as liberal reaction to the Catholic Church's condemnation of "modernism," and the politics of the Italian Risorgimento only reinforced the skewed interpretation of the Galileo affair as a prime example of the hostility of the Catholic Church to reason and science. How, so it was alleged, could the Church proclaim its pontiff to be infallible when at least two popes affirmed as a matter of faith the false position that the Earth didn't move? The legend of Galileo has roots in the Enlightenment and in the culture of positivism, but it achieved renewed currency in the late nineteenth century and casts a shadow of ignorance to this day.
There is no evidence that Galileo, when he acceded to the Inquisition's demand in 1633 that he formally renounce the view that the Earth moves, muttered under his breath, eppur si muove, but still it moves. What continues to move, despite evidence to the contrary, is the legend that Galileo represents science's fighting to free itself from the clutches of blind faith, biblical literalism, and superstition. Galileo and the Inquisition shared common first principles about the nature of scientific truth and the complementarity between science and religion. In the absence of scientific knowledge that the Earth moves, Galileo was required to deny that it did. However unwise it was to insist on such a requirement, the Inquisition did not ask Galileo to choose between science and faith.
Bibliography
The most complete recent history of the encounter between Galileo and the Inquisition is the work of Fantoli. For a good account both of Galileo the scientist and his encounter with the Inquisition, see Sharratt.
Professor Albert Van Helden of Rice University has established a world-wide web site on Galileo: http://es.rice.edu/ES/humsoc/Galileo/index.html; students should examine its offerings. In addition, students should consult the web site of the Institute and Museum of the History of Science in Florence: http://galileo.imss.firenze.it/museo/b/egalig.html
From time to time throughout the lectures we will be using references from the works listed below.
Brief Essays:
Blackwell, Richard J. "Galileo Galilei," in The History of Science and Religion in the Western Tradition: An Encyclopedia, edited by Gary B. Fergren, pp. 85-89 (New York: Garland, 2000).
Fantoli, Annibale. "Galileo and the Church," in Encyclopedia of the Scientific Revolution: From Copernicus to Newton, edited by Wilbur Applebaum, pp. 252-255 (New York: Garland, 2000).
Settle, Thomas B. "Galilei, Galileo," in Encyclopedia of the Scientific Revolution: From Copernicus to Newton, edited by Wilbur Applebaum, pp. 245-252 (New York: Garland, 2000). General Works:
Blackwell, Richard J. Galileo, Bellarmine, and the Bible. University of Notre Dame Press, 1991.
Blackwell, Richard J. Science, Religion, and Authority: Lessons from the Galileo Affair. Marquette University Press, 1998.
Drake, Stillman (ed.) Discoveries and Opinions of Galileo. Garden City, New York: Doubleday, 1957.
Fantoli, Annibale. Galileo: for Copernicanism and for the Church. (translated by George Coyne), second edition. Vatican Observatory Publications, 1996.
Finocchiaro, Maurice A. (ed.) The Galileo Affair: A Documentary History. The University of California Press, 1989.
Langford, Jerome. Galileo, Science, and the Church. Ann Arbor: The University of Michigan Press, 1966
Sharratt, Michael. Galileo: Decisive Innovator. Oxford: Blackwell, 1994.
More specialized studies:
Ariew, Roger. "Galileo's Lunar Observations in the Context of Medieval Lunar Theory," Studies in the History and Philosophy of Science 15, no. 3 (1984), pp. 212-227.
Brooke, John Hedley. Science and Religion: Some Historical Perspectives. Cambridge University Press, 1991.
Carroll, William E. "Galileo and the Interpretation of the Bible," Science & Education 8:2 (1999), pp. 151-187.
Feldhay, Rivka. Galileo and the Church: Political Inquisition or Critical Dialogue? Cambridge University Press, 1995.
Pederson, Olaf. Galileo and the Council of Trent. Vatican Observatory Publications, 1983. (Second edition, 1991)
Redondi, Pietro. Galileo Heretic. (trans. by R. Rosenthal). Princeton University Press, 1987 [Galileo eretico, Einaudi, 1983].
Van Helden, Albert (trans./ed.). Sidereus Nuncius or The Sidereal Messenger. Chicago: The University of Chicago Press, 1989.
Wallace, William A. Galileo and His Sources. Princeton University Press, 1984.
Galileo and the Renaissance
Peter Hodgson
1 Introduction
The life and achievements of Galileo form a subject of enduring interest. He is certainly one of the greatest scientists of all time and indeed has been called the founder of modern science. He showed that natural phenomena obey mathematical laws and thus laid the foundations of quantitative dynamics and used it to give the first accurate account of the motions of falling bodies and of projectiles. He improved the telescope and used it to discover the moons of Jupiter, the mountains on the moon, the phases of Venus and the spots on the sun. All this combined to throw doubt on Aristotelian cosmology, and to support the heliocentric theory of Copernicus. More than any scientist, he was responsible for initiating the transition from the Aristotelian science of the Middle Ages to the mathematical science of the following centuries.
This is more than enough to secure his fame, but he is more widely known because of his clashes with the authority of the Church. These are frequently presented as an archetypal struggle between reactionary theologians and a brave lone scientist, showing the irreconcilability of faith and science.
Like all scientists, Galileo learned from his teachers a way of looking at the world, the meaning of scientific explanation and the criteria of truth. These continued to exert their influence even as he was pioneering a new approach. He made many mistakes, sometimes holding on to his vision in the teeth of the evidence, as in his writings on the comets and his explanation of the tides. His assessment of his achievements is not always ours, and we must beware of interpreting him according to our own ideas and criteria.
Galileo lived at a critical moment in the development of science. According to the popular account, the first steps towards a scientific understanding of the world were made by the ancient Greeks. Their writings were inherited by the Muslim civilisation, and were transmitted to the new universities in the Middle Ages through translations made mainly in Spain. Thereafter the intellectual development of Western Europe was controlled by an authoritarian Church which prevented any independent thought or scientific development. It was only in Renaissance times that the authority of the Church was challenged by men like Galileo who insisted on the priority of experiment and observations over ancient texts. This is dramatised by the story that he dropped two balls of different masses from the leaning tower of Pisa and showed that, contrary to Aristotle, they reached the ground simultaneously. Thereafter science developed as a free and independent search for truth.
The reality is, of course, very different and highly instructive. The familiar story, still heard today, that there was no science worth speaking about in the long period from the time of the ancient Greeks to the flowering of genius in the Renaissance has long been disproved by modern scholarship. Galileo himself was not only a highly original scientist but remained throughout his life a devout Catholic (Pedersen, 1985). He had a sound grasp of theology and saw very clearly that the new knowledge of the world gained by the scientific method is in no way inconsistent with the teaching of the Church, since both come from God. He also saw that some of the new knowledge raised important problems of Scriptural interpretation that could be resolved within the context of traditional Catholic theology. It is now recognised that Galileo's views on the interpretation of Scripture are basically correct, and he was particularly anxious to prevent the tragedy that actually happened, namely the condemnation by the Church of a genuine scientific breakthrough. He was, however, over-confident concerning his scientific arguments which were still at that time inconclusive, at least to non-scientists. In view of the delicate theological questions raised by the heliocentric theory it was not unreasonable for the Church authorities to ask him to moderate his claims until a definite proof was forthcoming. The main protagonists were all motivated to defend the truth but they were strongly influenced by their intellectual backgrounds and did not lack personal character traits that exacerbated their misunderstandings.
To show how this drama developed it is desirable to consider briefly the principal influences on Galileo's thought, in particular the work of Archimedes and the natural philosophies of Plato and Aristotle. It should be mentioned that practically everything concerning the life and work of Galileo is the subject of controversy among professional historians, and so it is not easy to present an accurate and balanced account.
2 Cosmology in the High Middle Ages
Before considering the struggles and achievements of Galileo it is useful to sketch briefly the understanding of the physical world that existed before his time. The youthful Galileo was attracted to mathematics and avidly studied the works of Archimedes. His interest in hydrostatics was stimulated by Archimedes' solution of the problem of King Hiero's crown, which led to his first publication The Little Balance (1586). Nature, he realised, is written in the language of mathematics. Further stimulus came from his further experiments on the relation between musical tones and the length, weight and tension of strings. Galileo's work on the centres of gravity of solids led to his appointment to the chair of mathematics at Pisa. This emphasis on mathematics shows the influence of Plato, who was widely influential in the early Middle Ages, largely due to the writings of Augustine. Plato held that terrestrial phenomena are imperfect copies of abstract mathematical forms existing in the mind of God. Thus mathematical relations are only approximately realised in nature. It was Galileo's greatest achievement to show how nature follows mathematical laws, but he went beyond Plato in requiring exact correspondence, with the limits of experimental uncertainties.
In later medieval times, thinking about the natural world was dominated by the cosmology of Aristotle. Initially, his philosophy encountered some resistance, but it soon became generally accepted. Aristotle was a universal genius who made important contributions to physics, chemistry, astronomy, biology and medicine, as well as to philosophy, logic, metaphysics, politics and literary criticism, and organised this knowledge into a unified view of man and nature. His writings were immensely influential, and through the work of Aquinas and others were integrated with Christian theology. Over the centuries Aristotle's views were developed by numerous commentators, and so the Aristotelianism of the Middle Ages is not always the same as Aristotle's own views. Since Galileo had to contend with contemporary Aristotelians it is their views that are described here.
It is important to distinguish between the professional Aristotelians in the universities who infuriated Galileo by insisting on the literal text of Aristotle and refusing to listen to Galileo's arguments, and the open-minded Jesuits at the Collegio Romano who so strongly influenced the young Galileo in his formative years. These Jesuits followed Aristotle in many respects and taught 'a somewhat eclectic Thomism containing elements deriving from Scotist, Averroist and nominalist thought' (Wallace, 1984) that may be described as scholastic Aristotelianism. Thus Galileo, although he bitterly attacked the professional Aristotelians, particularly their views on mechanics and cosmology, retained throughout his life a basic adherence to Aristotelian natural philosophy.
Aristotle thought of nature as a process, an organism, and held that the main object of science is to see how it is related to man. The aim of science is to obtain certain knowledge by understanding the causes of natural phenomena. His cosmology was based on direct commonsense experience, and this is why it has such a strong appeal, even today. He emphasised the primacy of the senses, which takes precedence over any theory. Who can doubt that the earth is solid and immoveable, with the sun, the stars and planets moving around it? Do we not see the sun rising in the morning, moving across the sky and setting in the evening? The heavens seem perfect and unchangeable, in contrast to the earth, where all is changeable and corruptible. It is thus very reasonable to conclude that there are two kinds of matter, and correspondingly two kinds of natural motion: circular in the heavens and the linear motion of free fall or rise on the earth. In addition there is unnatural motion such as that of projectiles, which cannot be combined with natural motion.
The motions of the stars and the planets were studied by the astronomers and Ptolemy was able to describe them quite accurately by compounding circular motions in the form of cycles and epicycles. This was a purely mathematical description, and it was not maintained that the cycles and epicycles corresponded to anything real. In contrast, Aristotle sought a more physical cosmology in terms of real entities. The two approaches were finally unified by Kepler (Russell, 1975).
At the centre of Aristotle's cosmology is the immovable earth, and surrounding it a number of concentric crystalline spheres bearing the moon, the inner planets Mercury and Venus, then the sun and finally the outer planets Mars, Jupiter and Saturn. Enclosing all is the sphere of the fixed stars, and outside this nothing at all. There were differing views about the reality of the crystalline spheres; Aristotle believed that there are fifty-five in all, made of a pure, unalterable, transparent, weightless crystalline solid. The whole set of spheres rotates once a day, thus accounting for the diurnal motion of the sun and the stars. Seen against the background of the stars, the paths of the planets sometimes show a retrograde or looped motion, and this was accounted for by fixing the planets to secondary spheres linked to the main ones. The Aristotelian cosmology was thus able to give an account of all observable celestial motions, including the prediction of eclipses.
Guided by direct experience, Aristotle made a sharp distinction between terrestrial and celestial matter. Terrestrial matter is changeable whereas celestial matter is unchangeable. There are four types of terrestrial matter, earth, air, fire and water, and each seeks its natural place. Celestial matter is the quintessence (or fifth essence), pure and unchangeable, and naturally moves on the most perfect curve, the circle. On the earth, natural motion is linear: the falling of earth and water and the rising of air and fire. These motions accelerate because their cause becomes stronger as the body approaches its natural place. Unnatural motion, such as the flight of an arrow, requires the continuing action of a mover. Thus Aristotle's physics was based on direct observation and accounted for many natural phenomena in a reasonable and coherent way. As a result, it was widely accepted for two thousand years.
One of the weakest parts of Aristotle's physics is his theory of projectile motion. He had no concept of force and denied the notion of inertia. He rejected the theory of antiperistasis, attributed to Plato, that explained the continuing motion of a projectile after it has left the hand of the thrower by assuming that the medium in which the motion takes place is moved by the front of the projectile and comes round to the back and pushes it along. This is obviously false, as it implies that it is impossible to throw things against the wind. It also cannot account for the continuing rotary motion of a smooth sphere, or the flight of an arrow. Aristotle believed that since the motion is unnatural it requires the continued action of a mover, and this must be the medium. He therefore suggested that the thrower communicates motion to the medium and also the power to move.
Buridan, a fourteenth century philosopher, rejected this theory because it cannot explain the continuing motion of a spinning wheel and also because it is common experience that the medium resists the motion of the projectile. Instead, Buridan proposed that the thrower gives the projectile some impetus that carries it along after it has left the hand of the thrower. This is related to one of the arguments against the motion of the earth. According to Aristotle, a projectile thrown vertically upwards from a moving earth will fall behind and hit the ground west of its starting point, contrary to experience. The impetus theory, however, predicts that it retains an eastward impetus throughout its motion, and so returns to the same point as observed.
It was widely believed that the ancient Greeks had achieved the summit of knowledge; they knew essentially all that could be known, and so the answer to any problem could be found by scrutinising ancient texts, particularly those of Aristotle. The duty of a scholar is simply to understand, defend and teach Aristotle's ideas. Within this mindset the Aristotelians simply could not understand what Galileo was trying to do. To them, the world is a living organism that can be understood by experience and reason. For this, direct perception is all that is needed. They interpreted the world in terms of a close-knit system of purposeful behaviour, using organic categories and concepts like matter and form, act and potency, essence and existence. Thus the qualitative properties of things suffice to reveal their essences.
Galileo, in sharp contrast, said that it is an illusion to think that we can understand the essences of things; what we can and should do is to describe their behaviour as accurately as we can using mathematics, and then make experiments to test the validity of our ideas. Quantitative relations are the real clues to the unique, orderly, immutable reality. By establishing them we can find out how things behave, but not what they are. This seemed quite useless to the Aristotelians, who had a low view of mathematics: indeed, Aristotle 'left to mechanics and other low artisans the investigation of the ratios and other secondary features of acceleration' (Shea, 1977, p.142). To them, number, weight and measure have no philosophical significance; motion is interpreted in terms of purpose, and to this mathematics is irrelevant. They had no interest in accurate descriptions of the motions of projectiles, or in the mathematical description of levers and pulleys. Mathematics, they allowed, is an interesting game, but it can tell us nothing about the real world. Galileo, on the other hand, thought that their elaborate structure of abstractions in fact leads nowhere.
Since Galileo occupied a chair of mathematics his duty was to expound the works of Euclid and Archimedes. Thus he could be much freer in his criticisms of Aristotle than if he had been a member of the philosophical establishment whose main duty was to master and teach the works of Aristotle.
It is important to distinguish between Aristotle's general ideas concerning scientific method, his natural philosophy, and the way it was applied to particular problems. Aristotelian physics is an attempt to find the real structure of the world, deduced rationally from general principles, and this always remained Galileo's goal, though he went further than Aristotle by requiring a precise mathematical description of reality. Aristotle's cosmology included many statements about the heavenly bodies and detailed theories of familiar physical processes that have subsequently been found to be incorrect, but this does not necessarily falsify his natural philosophy. Thus although Galileo showed that many of Aristotle's views are incorrect, he did this within the framework of Aristotelian natural philosophy, and himself remained essentially an Aristotelian. Aristotle aimed to provide a rational account of the world, deduced from general principles. In the end, Aristotle's attempt was a heroic failure, largely because he greatly underestimated the difficulty of obtaining these principles, and also the value of precise measurement and detailed mathematical analysis.
Christian beliefs can easily be interpreted within the framework of Aristotelian cosmology. Hell is in the centre of the earth, and volcanoes provide evidence of its fires. Beyond the outermost sphere is the abode of God and the saints. Thus we can speak of the descent into hell and the Ascension into heaven. This imagery is lost in the heliocentric system. In addition, if the earth is just one of the planets, then is it not possible that people are to be found on other planets, and if so how can they be redeemed by Christ? The Aristotelian universe thus accommodated all that was known in a unified logical structure, and this accounts for its great power over the human imagination. To throw doubt on any part of it would be to threaten the whole and upset the well-established order of the universe.
3 The Birth of Modern Science
Modern science, by which we mean the detailed quantitative understanding of the material world expressed in the form of differential equations, is unique to our European civilisation. Nothing like it is found in the great civilisations of antiquity despite their impressive achievements in many other fields. It is commonly believed that modern science began in the Renaissance. Leonardo da Vinci, the great polymath, filled thousands of pages with accounts of dynamical principles, sketches of mechanical contrivances and other ingenious devices. But did this all spring from his own mind, or did he learn it from others?
Pierre Duhem was the first to study this question, and he found that all the results recorded by Leonardo were common knowledge in the High Middle Ages, and originated in studies by the Mertonian school of William Heytesbury, Richard Swineshead and John of Dumbleton in Oxford between 1328 and 1350 and the contemporary school of John Buridan and Nicholas Oresme in Paris. Many of their ideas are also found in the writings of John Philoponos in the sixth century.
The Mertonian school formulated the mean speed theorem, namely that in a given time a uniformly accelerated body traverses the same distance as a body moving with a constant velocity equal to that of the accelerated body at the mid-point of its motion. They had to express this verbally as the appropriate mathematical notation had not then been invented.
Duhem showed that science achieved its first viable birth in the High Middle Ages, when Christian theology provided for the first time in human history the essential beliefs about the material world that form the basis of modern science: that matter is good, orderly, rational, contingent and open to the human mind. The philosophers of the Middle Ages were independent thinkers who did not hesitate to differ from Aristotle if Christian beliefs, or reason or experiment required it. In particular, the Christian doctrine of the creation of the world out of nothing by God provided the stimulus to the Parisian philosophers like John Buridan to break with Aristotle and to formulate in a qualitative way the law of inertia, later to become Newton's first law of motion (Clagett, 1961). The writings of Buridan and his pupil Nicholas Oresme were widely diffused throughout Europe and provided the basis of our understanding of motion, the most fundamental problem of physics and hence of all science.
Oresme subjected many of Aristotle's arguments, including those against the motion of the earth, to a critical analysis, and showed that they are invalid. His purpose was simply to show that the earth could move; later on Galileo used the same arguments to show that the earth does move.
The impetus theory was also applied to celestial bodies, and Buridan supposed that when God created them he gave them the impetuses necessary for them to continue in motion. Thus, contrary to Aristotle, there is no distinction in this respect between celestial and terrestrial bodies. The breaking of Aristotle's distinction is necessary if the earth is to be considered one of the planets, making it possible for them all to follow the same dynamics.
The work of Buridan affected only the Aristotelian description of motion, and even in that area it was still possible to maintain the Aristotelian view by saying that now the mover is internal to the body but extrinsic. In other areas the vast system of Aristotle still dominated the intellectual scene. Furthermore, his philosophical concepts had been used by Aquinas and other theologians to express in a more precise way the whole of Christian theology. Dante's Divine Comedy describes his journey through the Aristotelian universe beginning on the earth and descending through the nine circles of hell that mirror the celestial spheres, then passing through purgatory and earth, through the celestial spheres to the throne of God in the highest sphere. This integration of Aristotelian cosmology and Christian theology is of great imaginative power and exerted a strong hold on the medieval mind. Since the whole conception is destroyed if the earth is allowed to move, we can appreciate the strong psychological opposition to such a suggestion. Anyone who challenged any part of the Aristotelian system could be sure to encounter strong opposition. The very idea that an upstart scientist with a leaden tube with lenses at either end could upset a vast philosophical system that had stood for two thousand years was just too preposterous to merit serious attention.
Since ancient times, there had been a few isolated astronomers who proposed a heliocentric cosmology, but the arguments against this view appeared to be overwhelming. If the earth rotates, the strong winds would blow everything off it. If the earth is moving around the sun, then the relative apparent positions of the stars must change, contrary to experience. Copernicus realised that these arguments are not conclusive, and that the heliocentric system had several advantages for computational purposes. These were gradually appreciated by astronomers, but to most people the heliocentric theory seemed just absurd. Some astronomers however, including Copernicus, became convinced of the correctness of the heliocentric theory, although they had no conclusive arguments in its favour.
Central to the whole debate is the question of why we believe, what are the criteria that we apply to judge whether a scientific theory is true or not. More fundamentally, how do we justify the criteria themselves? In Galileo's time, a theory was judged by the Aristotelian criterion: whether it gave an explanation in terms of causes. It was also required to 'save the appearances', that is to give predictions in numerical agreement with the experimental measurements. Finally, it had to be in accord with Scripture.
A related question is why a theory is accepted by some people and not by others. There could be a fundamental disagreement on the criteria to be applied, but there may also be inability to apply them due to lack of knowledge. Much scientific work depends on the correct interpretation of signs. Thus an elementary particle physicist can look at a bubble chamber or emulsion photograph and immediately identify the particles and processes responsible for them. It is possible for a sceptic to say that the whole photograph could be due to the chance alignment of unconnected bubbles or grains, but this suggestion would be derisively rejected by the scientist. The important point is that the photographs can only be interpreted by those with adequate knowledge. The same applies to medical X-ray photographs and radiographs. In addition to our technical knowledge we bring a whole range of beliefs about what is important and what is peripheral together with the experience gained by a wide range of observations. At that time science was considered a very minor activity, hardly worthy of attention, and certainly not comparable in importance with philosophy and religion. Thus the theologians who were not convinced by the arguments for heliocentrism were not necessarily obtuse or stupid, though the same cannot be said about those who refused even to look through Galileo's telescope.
4 Scientists in the Time of Galileo
Many of Galileo's actions, even those concerning his research, can only be understood in the context of the situation of scientists of his time. For those without private means, the only possibilities were employment by a university or by a rich patron. University positions were rather more secure, but they were poorly paid and it was necessary to give many lectures and to undertake long hours of private tuition. It was far more lucrative to hold a position in the entourage of a great prince, but this was continually dependent on the prince's favour. Moreover, most princes were more interested in their horoscopes and in practical matters such as the design of fortifications and the dredging of harbours. Sometimes they were interested in the scientific discoveries made by their scientists, but even then it was more as something that enhanced their prestige than for its intrinsic value. The scientists would often share the prince's table, and be expected to make entertaining and instructive discourse to impress the prince's guests. The scientist thus had to keep up a stream of interesting discoveries, and if he made a mistake this would tarnish the reputation of his prince and lead to his speedy dismissal. The favour of a powerful prince could be most prestigious and lucrative, but it was exceedingly precarious and could easily be lost.
In this situation, scientists were continually scheming to obtain the favour of a prince or, failing that, a university chair. It was normal to write fawning letters extolling their skills as an astrologer and as a designer of fortifications, and if they were successful they were likely to spend much of their time on such pursuits. They were inevitably the object of intense envy among other scientists, who would do their best to undermine and ridicule their work so that they fell out of the favour of the prince.
As an ambitious scientist, Galileo had to spend much time trying to find a position that would support his work, and his need was exacerbated by the demands of an impecunious family. His brother Michelangeiolo was an irresponsible spendthrift with a large family who was continually demanding money, and he was also expected to provide dowries for his two sisters Livia and Virginia, not to mention his own three children. These pressing needs go a long way to explain his pushy, aggressive and pugnacious behaviour. He had to fight to survive and he was not slow to round on his detractors with withering sarcasm. He did not scruple to advance his ambitions by political intrigue, and his enemies repaid him in kind. He used to the full his great powers as a writer in the vernacular to propagate his views, and indeed was the first scientific populariser.
5 Galileo's Life
Galileo was born in Pisa in 1564 and ten years later moved to Florence. He entered the university of Pisa in 1581 and decided to devote his life to physics. He developed a hydrostatic balance and worked on the centre of gravity of solids and this led to his appointment to the Chair of Mathematics at Pisa in 1589. There he wrote a book on motion (De Motu, 1592) summarising Aristotle's ideas, with critical comments and corrections. In 1592 he moved to a similar Chair at the university of Padua in the Venetian Republic. He wanted to return to his native Tuscany and to enjoy the extra freedom of a post in the entourage of the Grand Duke, and largely as a result of his astronomical discoveries succeeded in being appointed Chief Mathematician and Philosopher to Cosimo II, Grand Duke of Tuscany, in 1610. He remained in Florence for the remainder of his life, making several visits to Rome to publicise his work. His astronomical discoveries convinced him of the correctness of the Copernican system. His enemies criticised his Copernican views as contrary to Scripture, and he defended himself by writing an essay on the interpretation of Scripture. In 1616 the belief in a central sun was denounced as heretical, and the idea of a moving earth as erroneous in faith. These views were not to be taught or published. In 1632 he published his Dialogue on the Two Chief World Systems presenting arguments for and against the Copernican theory. In 1633 it was judged that by this work he has disobeyed the injunction of 1616 and so he was forced to recant and sentenced to spend the rest of his life confined to his villa in Florence. There he continued his scientific work until his death in 1642.
6 Galileo's Principal Writings
Galileo was acutely conscious of the importance of speedy publication to claim priority for his discoveries. In order to ensure this without prematurely revealing what he had found he sometimes resorted to the device of publishing an anagram. Then, when he had established the new result without doubt, he could reveal the meaning of the anagram. He wrote many books describing his work, some in reply to attacks on his ideas, or wrote formal letters to persons of distinction with a view to eventual publication. Finally there are longer, carefully-considered treatises that deal with a much wider range of material. It may be useful to list his principal writings in order of publication, together with references to available translations.
1. De Motu (On Motion) 1592. Considers the application of Archimedes' principle to motion in a medium. Summarises Aristotle's ideas on motion, with some critical comments. Translated with introduction and notes by I.E. Drabkin in Galileo Galilei on Motion and Mechanics. University of Wisconsin Press, 1960.
2. Le Meccaniche (On Mechanics) 1600. Summary of the statics of simple machines. Translated with introduction and notes by Stillman Drake in Galileo Galilei on Motion and Dynamics. University of Wisconsin Press, 1960.
3. Sidereus Nuncius (The Starry Messenger) 1610. An account of his discovery of the satellites of Jupiter and other astronomical discoveries. Translated with introduction and notes by Stillman Drake in Discoveries and Opinions of Galileo. Doubleday, Anchor Books, 1957.
4. Discorso . . . (Discourse on Bodies in Water) 1612. Describes experiments on floating bodies, with additional remarks on natural philosophy.
5. Letters on Sunspots, 1612. Critique of the views of Christopher Scheiner and a dispute over priority. Partly translated by Stillman Drake (see item 3).
6. Lettero alla Granduchessa di Toscana, Crestina di Lorena 1615. Summary of his view on the relation of theology to science. Translated by Stillman Drake. (see item 3). Finocchiaro, 1989, p. 87.
7. Discourse on the Tides, 1616. Finocchiaro, 1989, p. 119.
8. Il Saggiatore (The Assayer) 1623. Discussion of the nature of comets, and a general defence of scientific investigation. Partly translated by Stillman Drake (see item 3).
9. Dialogo . . . sopra i due Massimi Sistemi del Mondo, Tolemaico e Copernicano. (The Two Chief World Systems) 1632. Full discussion of the arguments for and against the Copernican system. Abridged translation and guide, Finocchiaro, 1997.
10. Discorsi a dimonstrazioni . . . (The Two New Sciences) 1638. Comprehensive discussion of the properties of materials and of terrestrial motions.
11. Dialogues concerning Two New Sciences. By Galileo Galilei. Translated by Henry Crew and Alfonso de Savio. Northwestern University Press, 1968.
12. Many letters and other documents are published by Finocchiaro, 1989.
Galileo the Physicist
Peter Hodgson
1 Introduction
Galileo had an insatiable curiosity about the world, and he believed that we can learn about it by careful observation and by accurate measurement. The results of measurements can be used to find relationships between the quantities measured, and these in turn can be used to develop and test theories about the behaviour of phenomena. He applied this method to a wide range of phenomena.
This combination of physical insight and mathematical description is the essential feature of modern science. It is not enough to have a physical description of a phenomenon, for this may be plausible but wrong. Neither is it adequate to have a set of mathematical rules that describe the measurable features accurately but give no physical insight, for there may be several sets that do this equally well. We only have a real understanding when both are combined. Even then, it may appear later that it is not a complete understanding, and that our knowledge has to be developed to cover a still wider range of phenomena.
It is not always easy to reconstruct exactly what Galileo did and his motives for doing so. It is extremely difficult, if not impossible, for us to think ourselves back into the mindset of the past, to know the mental background, to know what was taken for granted and what was implicitly denied, to understand what sort of arguments were accepted as valid, and the criteria used to separate truth from falsity. If we are to distinguish between genuine discovery and mere copying, we need to know what had already been discovered and how widely these previous discoveries were known. It is nearly always possible to find many precursors who had some partial or even quite accurate knowledge of what is claimed as a new discovery. Quite often the person recognised as the discoverer has taken some idea already known, rephrased it more clearly, demonstrated it by well-chosen experiments, and then has publicised it in a particularly arresting and cogent way.
In the case of Galileo, we have his extensive writings, but when we read them, especially in translation, we are faced with the problem of knowing just what the words mean. There is an ever-present danger that we use our present knowledge of physics to read into his experiments motives and ideas that were not his. The terms he used to describe his work are often translated by modern terms that have precise meanings that are unlikely to correspond to those in Galileo's mind.
The whole process of scientific discovery is mysterious even to the scientists themselves. When we are wrestling to understand some strange phenomenon we think about it for weeks and months, trying out one idea after another, until finally the light dawns. All these intermediate stages are soon forgotten, so it is impossible even for the scientists themselves to re-create the process. Certainly it is not present in the dry, logical account that is written up for publication.
These difficulties are particularly acute in the case of Galileo, for he inherited the mainly qualitative science of the Middle Ages, and was largely responsible for transforming it into the quantitative science we know today. The concepts used to describe motion, for example, were not clearly understood, and only gradually achieved the clarity we know today. The ideas defining the concepts of impetus and inertia developed over the years from the work of Buridan to that of Galileo, and the degree of continuity and discontinuity is still disputed.
In some cases it is doubtful whether Galileo actually performed the experiments he describes; they are more in the nature of thought experiments designed to clarify his ideas and to convince others of their truth. He undoubtedly made some experiments in order to understand the phenomena he was studying, and when he believed that he had attained this understanding he deduced the consequences for other situations which he had not studied.
Many of his experiments were technically difficult for him, since he did not have available the simple measuring instruments that we now take for granted. It was comparatively easy to measure distances, for example in his studies of falling bodies, but it was very difficult to measure short times at all accurately. The inevitable uncertainties in his results then made it more difficult to be sure that any relationship that he found is the only one possible. There is also the ever-present difficulty of removing or allowing for the effects of extraneous influences.
In the course of his work Galileo designed and made many ingenious new instruments, and frequently had them manufactured in his workshop for sale. An example is an early form of thermometer, and various quadrants and magnetic compasses. He also applied his knowledge of levers to simple machines and in his book on mechanics he described the windlass, the capstan, the screw and the Archimedean screw.
2 The Pendulum
Galileo's earliest biographer Viviani claimed that in 1582, when he was still a medical student in Pisa, he observed the motion of the swinging lamp in the cathedral. Using his pulse to measure the time of swing, he found that it is independent of the amplitude of the swing, providing that this is small. This is a rather surprising result, as it implies that it takes the same time for the pendulum to reach the nadir of its swing however far it is drawn aside before release. According to Viviani, this suggested to him that a pendulum could be used to measure the pulse rate. There is, however, no other evidence for this story, as Galileo first mentioned the isochronous nature of the pendulum in a letter in 1602. He also showed that the period of swing is independent of the material of the pendulum and that the period is proportional to the square root of the length of the string. Galileo also compared the swing of the pendulum with the motion of a ball that runs down one inclined plane and up another one opposite to it. In another investigation, he found that the times of descent are equal for all chords from the highest or to the lowest points of a vertical circle.
3 The Dynamics of Free Fall and of Projectiles
Galileo's earliest work on mechanics was in his De Motu of 1592, which is devoted to a discussion of the fall of bodies in media of different densities. In this work he was much influenced by the ideas of the Jesuits at the Collegio Romano, whose lecture notes he used extensively (Wallace, 1984). They held, with Aristotle, that the aim of science is the understanding of natural phenomena in terms of evident principles, and Galileo continued to accept this throughout his life. However he strongly opposed the arid textual Aristotelians found in the universities and it is against them that his polemics are directed. De Motu is largely a detailed analysis of the writings of Aristotle on the subject, and after about forty pages of discussion he exclaims: "Heavens! At this point I am weary and ashamed of having to use so many words to refute such childish arguments and such inept attempts at subtleties as those which Aristotle crams into the whole of Book 4 of De Caelo, as he argues against the older philosophers. For his arguments have no force, no learning, no elegance or attractiveness, and anyone who has understood what was said above will recognise their fallacies." (Drabkin, p.58). Later on he remarks that "Aristotle was ignorant not only of the profound and more abstruse discoveries of geometry, but even of the most elementary principles of this science"(p.70). A few pages later, discussing how projectiles are moved, he remarks: "Aristotle, as in practically everything that he wrote about locomotion, wrote the opposite of the truth" (p.76).
At that time, however, Galileo apparently thought that each of the cases he discussed was characterised by a constant velocity rather than a constant acceleration. He also accepted the current but false belief that if a light and a heavy body are dropped together, the light body will initially move more rapidly than the heavier, and so devoted several pages to ingenious arguments to explain why this happens. If indeed there is experimental evidence for this effect, it probably occurs because a heavy body has to be held more tightly than a light one, and so tend to be released a little later.
Galileo developed his views on motion throughout his life and his mature conclusions are described in his Discoursi of 1638 that in many respects is a precursor of Newtonian mechanics. The transition from medieval to Newtonian mechanics is thus largely due to him.
Since ancient times there was much discussion concerning the rate of fall of bodies towards the earth. Study of this natural motion was an essential preliminary to the discussion of the forced or unnatural motion of projectiles. Aristotle suggested that the speed of free fall V is proportional to the weight W of the body and inversely proportional to the resistance R of the medium. It is however not justified to conclude that V = kW/R, where k is a constant, since he also believed that the motion is accelerated: the velocity increases as the body approaches its natural place. Some medieval commentators suggested that the velocity is proportional to the distance d fallen, which would give V = kdW/R, but this connection was not made. The philosophers in Oxford and Paris in the fourteenth century succeeded in formulating the odd-number rule for the distances traversed by a uniformly accelerated falling body in equal times and this is equivalent to saying that the distance traversed is proportional to the square of the time, an achievement usually attributed to Galileo. The medievals however had no means of measuring acceleration and there was no justification for assuming that freely-falling bodies are uniformly accelerated, so this was no more than speculation.
These discussions went on for centuries without much progress because few if any actual experiments were made, the complications due to the resistance of the medium were not properly understood, and there was no clearly defined concept of acceleration. Thus what is now a trivial exercise for thirteen year olds was still a problem exercising the minds of the leading philosophers of nature.
It was not difficult to show that the above expressions for the velocity have unacceptable implications. They imply, for instance, that in a vacuum, when the resistance is zero, the velocity would be infinite (which, incidentally, provided Aristotle with his argument for the impossibility of a vacuum), and that two bodies of equal weight, falling side by side, would double their velocity if joined together. Furthermore, if the medium is denser than the body, the latter will rise and not fall.
We can now see that the resistance in a particular medium is not a well-defined quantity, as it depends on the size, shape and surface roughness of the body, and also on its velocity and internal motion. Even today this remains a very complicated problem. Since the resistance increases with the velocity it eventually equals the gravitational force so that after falling a certain distance the velocity becomes constant. This is obvious if we consider a metal ball and a feather falling through treacle, but also applies to free fall in air: the probability of survival of a cat that has the misfortune to fall out of the window becomes a constant for falls from more that about seven floors. The attainment of a terminal velocity was recognised by Galileo in his De Motu, although he thought that it is achieved more rapidly than it is.
To make any progress it is therefore necessary to consider the situation where the resistance of the medium can be neglected. Galileo noticed that bodies of different materials and shapes fall with very different velocities in dense materials, but at about the same velocity in air. This is supported by his experiments with a pendulum. He then conjectured that in a vacuum all bodies would fall with the same speed. The motion is then independent of the medium and of the nature, size and shape of the body. It is impracticable to make measurements in a vacuum but this is approximately true for the fall of smooth heavy spherical bodies falling through short distances in air. Ideally, we could make measurements in air of decreasing density and then extrapolate to zero density. There is always, as in most experiments in physics, a final leap from the best we can achieve to the ideal situation. Thus most of physics refers to an ideal Platonic world and not to the real world, a distinction that is seldom recognised, sometimes with disastrous results. This is directly contrary to Aristotelian physics, which is concerned with what normally happens, which implies that it is not possible to learn anything about natural motion by considering a situation, however ideal, that dos not actually exist.
Throughout his work Galileo made use of the principle that the laws of nature are simple. Thus the laws of motion must be expressed by simple formulae. In some contexts this led him astray, as when he rejected Kepler's conclusion that the planetary orbits are ellipses and not circles.
Galileo's views on motion went through several stages. At first, as described in De Motu, he believed that natural motion has a natural uniform speed proportional to the difference between the density of the moving object and that of the medium. As the effective density is diminished by the medium, so is the natural uniform speed. Non-natural motions are due to an impressed force, and this is responsible for the initial acceleration. These views constituted a coherent philosophy of motion, but Galileo could not find a single example of this uniform motion and so concluded that acceleration is a feature of all motion. He considered the possibility that the velocity is directly proportional to the distance, but soon rejected this possibility. Then, from the mean speed theorem, which implies that the acquired velocity is proportional to the time taken, he deduced that the distance covered is proportional to the square of the time taken. We can obtain this result more easily using Newton's notation: x' = g, x" = gt, x = gt2/2.
There has been some controversy about whether Galileo actually performed any experiments related to free fall. It is suggested that since he already knew that the distance travelled is proportional to the square of the time the actual experiment was undertaken to confirm that result. It is certainly true that many of Galileo's earlier statements about motion were the result of thought experiments. He believed that ideas suggested by simple observations are often misleading, and that they must be tested by mathematical reasoning. Later on he realised the need for carefully planned experiments.
Galileo soon found that it was very difficult, if not impossible, for him to measure with sufficient accuracy the time taken for bodies to fall. He therefore hit on the ingenious idea of timing them as they rolled down planes inclined at different angles. The times to be measured are much longer, and so could be measured more accurately. He could make measurements for a series of increasing angles, and then extrapolate to find the rate for free fall. The experiment is indeed quite practicable, as shown by Settle (1961).
Even then it was not easy to measure the times. Eventually he did this by weighing the amount of water that spurted out of a pipe from a small hole near the bottom of a large jar of water. He stopped the hole with his finger, removed it when the ball started and stopped the flow when the ball had traversed the prescribed distance. After many hundreds of trials he found that the distance traversed is proportional to the square of the time taken for all angles, although of course the constant of proportionality varied with the angle. Although Galileo did not recognise this, it was not possible to extrapolate the constant of proportionality to obtain that for free fall, what we now know as the acceleration due to gravity. because when the balls rolled down the plane some of their potential energy was converted into rotational energy, and not to kinetic energy, to use the modern terminology. A simple calculation shows that this does not invalidate the time squared law, but it reduces the value of the acceleration by a factor 5/7.
Galileo also made further studies of motion that do not require time measurements. He let balls roll down an inclined plane, and at the end of the plane they were deflected horizontally and then allowed to fall freely until they hit a horizontal plane. The time squared law implies that the path of free fall is a semi-parabola, so that by seeing how the length and angle of the inclined plane was related to the point of contact on the horizontal plane Galileo could verify the correctness of the law.
One of the most familiar stories about Galileo, also due to Viviani, is that he dropped two different weights from the top of the leaning tower of Pisa and that, to the dismay of the watching Aristotelians, they hit the ground at the same time, thus disproving Aristotle's law. If ever he did the experiment, however, and if he succeeded in releasing them at exactly the same moment, which is not as easy as it sounds, careful observation would have shown that, due to air resistance, the heavier body would have hit the ground slightly before the lighter body. This is still quite different from the proportionality given by Aristotle.
Galileo also considered the fall of bodies in a medium. As an example he chose two balls of lead and ebony. Supposing lead to be ten thousand times and ebony to be a thousand times as heavy as air he concluded that if they are allowed to fall from a tower two hundred cubits high, the lead ball will outstrip the ebony ball by less than four inches. Since a cubit is about 20 inches, he is saying that the lead ball outstrips the ebony by less than one thousandth of the height of the tower. This follows approximately from the assumption that the velocity of the lead ball is reduced by a factor (1 - 1/10,000) and that of the ebony ball by a factor (1 - 1/1000). No justification for this conjecture is given, and it is clear that the result is hypothetical and not the result of any experiment.
Physicists, and Galileo was no exception, are sometimes prone to imagine that they have such a firm grasp of a particular phenomenon that they can confidently say what is going to happen without making any experiments. Frequently their confidence is justified, especially when they are making qualitative predictions, but sometimes they are wrong. Many instructive examples could be given from the history of science. Quantitative speculations, like that mentioned above, are much more shaky.
When he came to consider the motion of projectiles, Galileo was faced with the problem of combining the unnatural motion due to the action of the thrower with the natural motion due to the tendency of all bodies to move towards their natural place. Aristotle believed that they cannot be combined so that, for example, the shot from a gun is first impelled by its unnatural motion along a linear path in the direction of the barrel and then, when that motion is exhausted, begins to fall vertically downwards following its natural motion towards the earth. Galileo began by considering the simple case when a body is thrown horizontally; the natural motion is its vertical fall according to the time squared law and the unnatural motion is the horizontal motion that, as will be shown below, has a uniform velocity. He enunciated the important principle that these two motions can be combined vectorially, so that the resulting motion is the sum of the two independent motions, which is a semi-parabola.
Galileo considered motion on a horizontal plane as a limiting case of his inclined plane experiments. He found that if he had two inclined planes arranged so that the ball rolls down one and then up the other, then whatever the angle of the second plane the balls always rolls up to very nearly the same height as it had on the first plane when it was released. Now reduce the angle of inclination of the second plane until it is infinitesimally close to the horizontal; in the limiting case it will roll on forever with constant velocity. In practice it will eventually come to rest due to air resistance and friction, but as always Galileo abstracted from such non-essential disturbances. By a horizontal plane he evidently meant a plane that is parallel to the earth's surface, which is essentially flat for practical purposes when small distances are involved.
4 Hydrostatics and Floating Bodies
The ancient Greeks discussed the reasons why some bodies float and others sink, and how this depends on their shapes and densities. The achievement of Archimedes in devising a method to determine the presence of base metal in the king's golden crown is well-known. It is surprising to us that such simple problems, now easily understood by children, were the subject of impassioned and confused debate for centuries by highly intelligent men. This reminds us that it was far more difficult than we think to establish the conceptual framework within which such problems can be clearly discussed, and that this was largely due to the work of Galileo.
Galileo became involved in such problems during a series of philosophical discussions held in the villa of Filippo Salviati near Florence. An Aristotelian philosopher maintained that the action of cold is to condense, but Galileo said that since ice is lighter than water the action of cold is to rarefy. The philosopher replied that ice only floats because of its shape, whereas Galileo maintained that it floats whatever its shape. At the next meeting one of the philosophers, Ludovico delle Colombe, showed that thin plates of ebony float on water, whereas other shapes sink, thus showing that whether bodies float or sink depends on their shape. This discussion, initially friendly, soon escalated into a bitter feud, and Galileo was told by the Grand Duke to avoid controversy and to confine himself to written comments. So he wrote his Discourse on Floating Bodies, which was published in 1612.
Following Archimedes, he said that the upward force on a body immersed in a fluid is equal to the weight of the displaced fluid, i.e. dvg, where d is the density of the fluid, v the volume displaced and g the acceleration due to gravity. Similarly the weight of the body is DVg. Thus the net upward force is
(dv - DV)g. If the body is completely submerged, then V = v, and this becomes
(d - D)Vg. Thus whether a body sinks or rises depends on the difference between the densities of the body and the fluid.
According to this, ebony should always sink, as it is slightly denser than water. Why then do thin plates of ebony float? Galileo did notice that such plates depress the surface of the water, thus effectively increasing the volume of fluid displaced sufficiently to keep the ebony floating. However the full explanation requires the concept of surface tension, which was not at that time known.
These considerations mark an important advance on the ideas of Aristotle, who said that there were two kinds of motion, one due to a natural inclination to rise, like fire, and the other an inclination to fall, like heavy bodies. Galileo showed that whether a body rises or falls depends only on whether it is less dense or more dense than the surrounding medium, so that all bodies obey the same law. This progressive unification of apparently different phenomenon is characteristic of the advance of science.
5 The Atomic Constitution of Matter
Galileo's discussion of floating bodies led him to speculate that motion through water is rather like pushing oneself through a crowd or thrusting a stick into a heap of sand. He thus thought of liquids as composed of multitudes of tiny particles, too small to be visible.
He also thought that fire provides evidence for the atomic constitution of matter. Democritus maintained that broad plates are able to float by heat-particles rising in the water, whereas narrow plates sink because there are too few such particles impinging on them. Aristotle rejected this argument, saying that if it were true heavy objects would float more easily in air than in water. Galileo considered this argument to be incorrect because bodies weigh more in air than in water,and also there is no reason to suppose that fire-atoms move more rapidly in air. He suggested that a thin, broad plate of a material sightly denser than water be placed on the bottom of a vessel filled with water. On heating, the fire atoms, if they can support it on the surface, should be able to raise the plate. However this does not happen, and so he concludes that the fire atoms are not able to provide the full explanation for the floating of such bodies (Shea, p.28).
Galileo also considered evaporation and boiling, and said that he could see millions of small spherical globules of fire rising through water when it is heated, and passing through the surface into the air. He rejected the view that the globules are water changed by the fire into vapour because the level of the water never falls, however long it is boiled.
This is an instructive example of a failure of Galileo's method. He believed that he could, on the basis of a few experiments, understand the principles governing the behaviour of a particular phenomenon. After that, simply by deductive reasoning, he could say with confidence what would happen in a large variety of circumstances that had not been experimentally investigated. Often this method worked well, but if the understanding is in any way faulty, it inevitably leads to false conclusions. It is very easy to see only what we want to see, and of course we want to see behaviour in accord with our own theories.
His speculations about heat were more successful when he said that it is not the fire-corpuscles that give the sensation of heat but their motion, and thus explains how a stone or a stick can be heated by rubbing. This changes them into 'very subtle flying particles', or perhaps releases fire-corpuscles, and these produce the sensation of heat.
Galileo also considered the strength of materials. He wanted to understand why it is that when machines are constructed "the larger machine, made of the same materials and in the same proportions as the smaller, will correspond to it with perfect symmetry in all respects except that of strength and resistance to breakage; the larger it is, the weaker it will be". This phenomenon is also notable in the animal world: an elephant is more massively constructed than a gnat. At first sight this seems to reveal a discrepancy between matter and geometry, between physical and mathematical divisibility. To understand this requires a consideration of the strength of materials, and this led Galileo to think about the ideas of continuity, the vacuum and the atomic structure of matter.
It has been suggested by Redondi that Galileo's atomistic explanation of sensory perception has heretical implications for the dogma of Eucharistic transubstantiation, and that this is the real reason for his trial and condemnation. This suggestion is based on a single document that in fact has nothing to do with the trial, and so there is no basis for Redondi's claim (Carugo and Crombie, 1988).
6 The Velocity of Light
In his Dialogue Concerning Two New Sciences Galileo describes an experiment to determine the velocity of light. Two people, each with a lantern, stand several miles apart. The first uncovers the lantern and then immediately the light is seen by the second he uncovers his lantern. The first then measures the time that elapses from the moment he uncovers his lantern to when he sees the light of the second lantern. This time, divided by twice the distance between the two people, give the velocity of light. However it was found that the time was immeasurably small, and so the experiment failed. We now know that the velocity of light is so great that such experiments are bound to fail.
An interesting sequel is that the first reliable measurement of the velocity of light was made by Romer by observing the eclipses of the satellites of Jupiter. These were found to occur rather later than expected when Jupiter was far from the earth, compared with the times when Jupiter was near. This is due to the time taken by the light to travel from Jupiter to the earth, and from this the velocity of light was determined.
Galileo also tried to develop a method of determining longitude at sea by observations of the satellites of Jupiter. Although this is possible in principle, it was found to be impracticable because at that time it was not possible to measure the time at sea with sufficient accuracy. This was achieved later by Harrison. The method has, however, proved useful in surveying on land.
7 Galileo's Scientific Method
When he was a young professor in Padua, Galileo was strongly influenced by the writings of the Jesuits teaching at the Collegio Romano, particularly Menu, Vella, Rugierius and Vitelleschi, and he based his lectures on their work. These Jesuits accepted Aristotle's definition of science and treated logic and physical questions in a realist way, following Aquinas. This formed the solid basis of Galileo's subsequent work (Wallace, 198. 1984, 1986)
Galileo realised more clearly than anyone before him that the primary task of the physicist is to understand the world as it is, to penetrate behind the apparent complexity of phenomena to the often surprisingly simple reality beneath. Thus when he considered freely falling bodies he wanted to establish the laws obeyed by all bodies of whatever shape or material. He therefore considered fall in a vacuum but, as this cannot be realised in practice, he chose the best approximation, namely the fall in air of smooth hard balls. The physicist is almost never able to make an experiment in an ideal situation, so it is necessary to consider all the unwanted influences that could affect the final result, and to allow for them. This often requires a subsidiary experiment to study and quantify these influences. This evaluation of perturbing effects is a vital component of the art of scientific investigation.
Galileo also distinguished between primary and secondary qualities. He pointed out that all bodies have a shape and a size, that it is in a particular place at a given time, that it is moving or stationary and so on. These are primary or essential qualities and cannot be separated from the body. On the other hand there are other secondary qualities such as colour, taste and smell that, although grounded in the properties of the body, are in themselves sensations that exist only as they are perceived by the observer. Scientific research is thus essentially concerned with studying the primary qualities of bodies.
In his research, Galileo combined the insights of Aristotle and Plato, and went beyond them. Like Aristotle, he insisted on the primary importance of experience, of the knowledge that comes to us through the senses. This knowledge, however, cannot be taken at its face value, but must be tested by combining it with other experiences and uniting them all by a general principle or theory. This theory cannot be deduced from the experiences; it is a creation of the human mind. The theory should not only agree with the original experiences, but usually also predicts a range of other experiences that enable it to be tested. In order to specify these new experiences we need not only Aristotelian logic but also mathematics. The theory and the mathematics refer to an ideal world and are thus Platonic in nature. Theories are tested by making experiments, and the conditions are chosen so as to be as close as possible to the ideal situation. If the results disagree with the theory, then the theory must be modified so as to be consistent with the new experiences and then tested again. It is not necessary to make a very large number of experiments; due to the uniformity and rationality of nature a few well-chosen experiments suffice.
There are many practical difficulties in carrying out this programme. What experiences do we start with? Usually this is indicated by an existing theory. If this has a mathematical character it is necessary not only to observe but also to measure. The construction of the theory depends on the insight of the scientist and cannot be specified by a set of rules; it may therefore be wrong in a fundamental way or, more frequently, it may be inadequate in one respect or another. Its consequences are likely to be very extensive, and it is not easy to choose the ones that make the sharpest test and yet are relatively easy to carry out. If there is a disagreement, is it due to a defect in the experiment or does it show a real defect in the theory? If the latter, then how should the theory be modified, and so on.
As the theories become more sophisticated and agree with a wide range of experience they may be said to give genuine, though still limited, knowledge about the world. As confidence grows, it become less necessary to make experimental tests, and this is certainly true of the laws of motion. However it always remains possible that new experiences show inadequacies in the theory that require it to be modified. There are many examples of this in the history of science.
Galileo was thus neither a pure Aristotelian nor a pure Platonist (McTighe, 1967). He could with justice claim that he was a better Aristotelian than many of the professed Aristotelians that criticised him. He, like Aristotle, observed nature, and did not seek the answer to questions only in books. If Aristotle had been able to look through a telescope he would have certainly modified his views. Likewise Galileo was a good Platonist by his stress on the importance of mathematics, which Aristotle undervalued. However Plato considered that the material world is an imperfect copy of the ideal world, whereas Galileo believed in the possibility of an exact mathematical description.
Galileo never formulated a fully-articulated theory of the scientific method; indeed this is still the subject of controversy today. He was a pioneer with a vision of the future and had to develop his tools as he tackled new problems. He was primarily interested in solving problems, not in explaining the methods he used to solve them, which he made up as he went along. Yet in so doing, he was inevitably throwing doubt on the traditional Aristotelian natural philosophy, and this could have the most far-reaching and serious consequences. Structures of thought are linked together far more tightly than is generally supposed, so that it is not possible to modify one section without affecting the others. This was clearly seen by many of Galileo's opponents and ensured their opposition, even if they were unable to mount any effective criticism of his actual scientific work.
Galileo the Astronomer
Peter Hodgson
1 The Invention of the Telescope
In 1609 Galileo heard that a Dutch optician, Lippershey, had found that if he put two lenses at either end of a tube and looked though it, distant objects appeared much closer. Many of these telescopes were made, but as their magnification was low and the images rather blurred they were regarded more as interesting toys than as objects of practical value. Galileo, however, immediately realised the importance of this invention and how he could use it to further his career by offering it to the Venetian State. He was alarmed to learn that a Dutchman was already in Venice hoping to sell his telescope to the Doge. He alerted his friend Sarpi, who succeeded in preventing the Dutchman from obtaining an audience with the Doge, and frantically set to work to make a telescope for himself. He fitted two lenses at either end of a lead tube and indeed found that it magnified distant objects. Later on he said that he succeeded in making a telescope in a single day, but this seems very unlikely. It takes a long time to grind a lens, and it is unlikely that he already had commercial lenses available or that they would be of sufficient quality. By a process of trial and error he went on to make a series of telescopes of increasing magnification and technical excellence. As soon has he had made a good telescope he arranged with the help of Sarpi to have an audience with the powerful Doges of Venice, who were impressed by its value to the Venetian Navy. Astutely, Galileo presented his best telescope to the Doge as a gift and, not to be outdone, the Doge's Senate soon after voted to double his salary, to reappoint him for life, and to give him a large bonus. Subsequently he made many more telescopes and presented them to many eminent friends and powerful princes. This story illustrates very well Galileo's ruthless opportunism and technical genius. Certainly his best telescopes were far superior to any others, and he made sure that their merits were widely recognised and that his career benefitted.
2 The Discovery of the Moons of Jupiter
He turned the telescopes to the heavens, and was rewarded by a series of outstanding discoveries. He looked at the planets, and noticed that there were one or two stars on either side of Jupiter, almost in a line. On subsequent nights he found that the stars had moved relative to Jupiter, and that their number changed. He realised that he was seeing four of the moons that orbit Jupiter just as the moon orbits the earth. By observing them for several weeks he was able to determine their periods of rotation. He called them the 'Medicean stars' in honour of Cosimo de Medici, and published an account of his discovery in a pamphlet called Sidereus Nuncius, or Starry Messenger. At first his discovery was ridiculed, but most people were soon convinced when they looked through one of his telescopes. He presented telescopes to several powerful princes, and they naturally asked their own astronomers to examine them and assess their merits. By this clever move, the astronomers were forced to examine his claims, whether they believed them or not, and indeed they soon endorsed them.
Galileo was particularly excited by this discovery because it provided an example of several moons orbiting a planet, very similar to Copernicus's suggestion that the planets orbit the sun. It did not, of course, prove Copernicus's theory, but showed that it was not necessary for everything to rotate about a single centre, and also answered those who said that if the earth moves it will lose its moon.
This spectacular discovery made Galileo famous throughout Europe, and he followed it by a whole series of new observations. He found hundreds more stars in the familiar constellations, and showed that the Milky Way is made up of thousands of individual stars. He turned his telescope to the moon and observed the circular craters that we now know are due to the impact of meteorites. By observing the behaviour of the shadows of their edges as the moon waxed and waned he was able to show that they had a central depression surrounded by a high rim, and estimated that they were about four miles high, a reasonably accurate value.
This discovery was important because Aristotle had said that the heavenly bodies were perfectly spherical, with no rough surfaces. The Aristotelians tried to explain Galileo's observations by saying that the moon is completely surrounded by a smooth transparent shell that covers all the craters. Galileo sarcastically replied that he would believe this if they would allow him to cover the moon with high and transparent mountains.
These new results supported previous observations of changes in the skies. Thus in 1604 there appeared a new star that excited great public interest. Galileo gave three lectures on the phenomenon, admitting that he was not at all sure that it was really a star; for all he knew it might be due to the condensation of vapours in faraway space. Studies of its parallax showed that it was much further from the earth than the moon and so provided another example of imperfections in the celestial realm.
These new discoveries were highly uncongenial to the Aristotelians, who redoubled their efforts to discredit his work, maintaining that what he saw was due to imperfections in his telescopes. Galileo defended himself vigorously, first developing the opposing views and supporting them by real arguments, and then demolishing the whole structure with undisguised relish.
His discoveries were soon accepted by other astronomers, particularly by the Jesuits of the Collegio Romano, who became very supportive of his work. Galileo visited Rome in 1611 to lecture on his discoveries and was feted by them.
In the same year Galileo studied the planet Saturn, and found that sometimes it had the appearance of three stars in the form of a larger central body with two equal satellites on opposite sides. At other times these satellites disappeared. His telescope was not sufficiently powerful to show that this was due to Saturn's rings seen from various angles, as was shown by Huygens in 1657.
In 1611 Galileo observed the sunspots, that provide another example of imperfections in the celestial realm. He correctly surmised that they are clouds of vapour on the sun's surface, and by observing their motion was able to deduce that the sun rotates with a period of about a month. He did not discover the sunspots; indeed they had been known to the Chinese for centuries, and other European scientists had also noticed them. Among these was the Jesuit astronomer Scheiner who sent his observations to Mark Welser, who published them anonymously under the pseudonym Appelles. In order to maintain the incorruptibility of the heavens Scheiner suggested that sunspots are little planets orbiting the sun. Welser sent Scheiner's letters to Galileo asking for his opinion, and Galileo replied at some length. He had little difficulty in demolishing Scheiner's theory, but did so in a very courteous way. Nevertheless Scheiner was very offended, probably because he mistakenly thought that Galileo had him in mind when he criticised one of his opponents without mentioning him by name; this was to cause Galileo much trouble later on.
To disprove Scheiner's theory Galileo observed that the sunspots are approximately circular when they are near the centre of the sun's disk and progressively become more elliptical as they approach the edge. This is just what would be expected if they are situated on the surface on the sun; if they were spherical planets they would keep the same circular shape as they moved around the sun. Galileo also showed that he could explain the velocities of the spots, and found that they are inconsistent with Scheiner's theory.
Galileo also saw that the trajectories of the sunspots change through the year: they appear to move in a straight line only at six-monthly intervals and at other times they appear to move in concave or convex arcs. This is what would be expected if the sun's axis is tilted with respect to the plane of the earth's orbit around the sun. While it is true that these motions could be described in the reference frame of a stationary earth, this would require the attribution of four independent motions to the sun (McMullin, 1967, p.40), whereas the motions are described quite naturally on the heliocentric theory. Thus although this does not amount to a strict proof, Galileo was justified in using the motion of the sunspots as an argument in favour of the heliocentric theory.
3 The Heliocentric Theory
Copernicus' book De Revolutionibus, putting forward the heliocentric theory, remained Aristotelian in all except its central idea, and is written so that, apart from some introductory sections, it can only be understood by professional astronomers. Initially, Copernicus was concerned, like Ptolemy, to find the best way to calculate the motions of the planets, and used the heliocentric hypothesis as a calculating device. As the work proceeded, he found that it accounted naturally for many observations that could be fitted by the geocentric model only by making specific assumptions in each case. Eventually he came to believe that the Copernican theory is true. As Galileo remarked in a letter to Mgr Dini in 1615, "From many years of observation and study, he was abundantly in possession of all the details observed in the stars, for it is impossible to come to know the structure of the universe without having learned them all very diligently and having them very readily available in mind; and so, by repeated studies and very long labours, he accomplished what later earned him the admirations of all those who study him diligently enough to understand his discussions. Thus, to claim that Copernicus did not consider the earth's motion to be true could be accepted perhaps only by those who have not read him, in my opinion; for all six parts of his book are full of the doctrines of the earth's motion, and of explanations and confirmations of it". (Finocchiaro 1989, p.60).
Copernicus was highly regarded by the professional astronomers, and they realised the many advantages of the new system. Many of them began to use his methods, even if they continued to reject his heliocentrism.
Subsequently, Tycho Brahe proposed a new cosmology, in which all the planets revolve around the sun, which in turn revolves around the earth. Providing the spheres of the fixed stars is sufficiently far away, this is mathematically equivalent to the Copernican heliocentric system. It was adopted by many astronomers as a way of using the ideas of Copernicus while avoiding the apparent absurdities of a moving earth.
The Copernican system, particularly when refined by Kepler's ellipses, provided a more accurate way of analysing planetary motions, but at the expense of the physical explanation provided by the Aristotelian spheres. A whole new set of questions arose and demanded answers: why do the planets move in elliptical orbits, what keeps them going around the sun and so on. Kepler suggested that the sun emits rays that push the planets around, and that the ellipticities of the orbits are due to magnetic effects. This idea did not fit the data and was soon discarded, but it contained the germ of the idea of forces that was eventually to provide the solution. Galileo ignored this possibility, considering Copernican heliocentrism as the only alternative to Aristotelian geocentrism.
The heliocentric theory provided natural qualitative explanations of several phenomena, such as the retrograde motions of the planets, the phases of Venus and the angular closeness to the sun of the inner planets Mercury and Venus. In the Ptolemaic system these observations were included by the special choice of the parameters of the ellipses. Although some astronomers including Copernicus became convinced of the correctness of the heliocentric theory, they had no conclusive arguments. It was easy to defuse the opposition likely to be encountered by the heliocentric theory by maintaining that it was just a convenient mathematical scheme with no pretensions to reality. The Lutheran theologian Osiander, who saw the manuscript of Copernicus through the press, inserted an anonymous preface to this effect, without Copernicus knowing.
The professional astronomers were well aware of the large number of minor improvements that had been made in the Ptolemaic system over the previous centuries without significantly improving the fit to the unsatisfactory ancient data and which proved quite unable to fit the greatly improved data of Tycho Brahe, increasingly turned to the Copernican theory as the basis of their calculations. Many of then still rejected the heliocentric theory as a real account of celestial motions and used the Copernican theory simply as a method of calculation. The astronomers gradually improved the Copernican theory and found that it is much more tightly constrained that the Ptolemaic theory, so that it is not possible to adjust the parameters of the planetary orbits independently of each other. And so, gradually, impelled by their very practical concerns, the belief of the professional astronomers changed. By around 1616, the time of the Church's first action against Galileo, the case for Copernicanism was respectable but still weak, but by the time of his recantation in 1633 the tide had turned and geocentrism was almost a lost cause. According to Kuhn, 'by the middle of the seventeenth century it is difficult to find an important astronomer who is not Copernican; by the end of the century it is impossible'. It took much longer for heliocentrism to be generally accepted; Milton, for example, in his great work treated the Ptolemaic and Copernican systems on an equal footing.
It is important to recall that not one of the arguments for Copernicanism was conclusive. Galileo's favourite argument from the tides is fallacious. Bellarmine said that if the heliocentric theory was proved correct (and by proof he meant, following Aristotle, certain knowledge through causes) then it would be necessary to study carefully how it could be reconciled with Scripture. However the proofs that first convinced the astronomers were only accessible to them; it is the cumulative effects of a large numbers of indications, individually not coercive, the unity of indirect reference akin to the illative sense of Newman. This may also be described as the interpretation of signs, which can only be done by the prepared mind. When eventually the definitive proof of the heliocentric theory came two hundred years later with the measurement of stellar parallax by Bessel in 1838, the battle was long over, and it is doubtful if there was any great stir among either scientists or theologians.
If it had been purely a matter for astronomers, the Copernican view would probably have gradually prevailed, without drama. However the prestige of Aristotelian cosmology, and especially its integration with Christian theology, made this impossible.
Galileo's discoveries brought the whole heliocentric debate out of the domain of the professional astronomers and into the area of public discourse. Using his telescopes, people could see the evidence for the celestial phenomena that were contrary to the Aristotelian view such as the mountains on the moon, the sunspots, the moons of Jupiter and the phases of Venus. None of this proved the heliocentric theory, but by weakening the Aristotelian cosmology it made it more worthy of consideration. Telescopes soon became very popular, and Galileo had to make many more to satisfy popular demand. At the same time he announced his discoveries in well-written booklets in the vernacular language. In contrast to the impenetrable tome of Copernicus, these were immediately accessible to non-professionals. Sure of his position, Galileo poured scorn on his opponents, thus further inflaming the opposition. Heliocentrism became popular among those who opposed Aristotle for other reasons, even if they had little understanding of the astronomical arguments.
Galileo maintained that he was more faithful to Aristotle than the Aristotelians. Aristotle believed in the importance of observation and reason, and Galileo believed that if he had the opportunity to look through a telescope, he would soon be convinced by what he saw, and would revise his cosmology accordingly. Galileo had nothing but contempt for those who looked for the truth about the physical world only by searching through musty old texts instead of opening their eyes to the world around them.
4 The Motion of the Earth
The central theme of Galileo's scientific endeavours was his continuing efforts to prove that the earth moves in two distinct ways: its daily rotation on its axis and its annual motion around the sun. These themes link together his astronomical discoveries and his work on terrestrial dynamics.
To achieve this aim he had to overcome three distinct sets of obstacles. Firstly the commonsense conviction based on direct experience that the earth is immoveable, secondly the opposition of the Aristotelian philosophers, whose whole view of the universe was centred on a stationary earth, and thirdly the belief of many theologians that a moving earth was contrary to Scripture.
The commonsense belief in an immoveable earth can be supported by rational arguments. If the earth is moving with the speed necessary to carry it round the sun, then surely the high winds would demolish all buildings and blow everything away. It is well known, it was said, that an object dropped from the high mast of a ship falls nearer to the stern because the ship moves while it falls, and therefore the effects due to any motion of the earth should be even more marked. Galileo responded by showing that this statement is false; the object lands at the foot of the mast, and this is because the object shares the forward motion of the ship all the time. He then pointed out that if one is in a closed cabin in a steadily-moving ship we can play ball and jump around just as we could if the ship were stationary. Indeed there is no way of finding out if the ship is moving or not. We can experience the up and down motion of the waves, and any changes in the forward speed, but these are all accelerations. This absence of any effects due to a uniform velocity is known as the principle of Galilean relativity. Thus the absence of any such effects is no argument against the translational motion of the earth.
It may be remarked that these arguments are strictly true only for rectilinear motion. On the earth, however, the situation is different because the objects on the earth's surface have circular trajectories, although the difference is very small for short distances. Thus the top of the mast is moving slightly more rapidly than its foot, due to the earth's rotation, and this implies that an object dropped from the top of the mast will hit the deck a small distance to the east of the base. The effect is very small but not negligible, and was indeed first detected by G.A. Guglielmini in Bologna in 1789 and confirmed by J.F. Benzenberg in 1802 and 1804, and by F. Reich in 1831. The most accurate study was made by E.H. Hall at Harvard in 1902, and he found a deviation of 1.50+0.05 mm to the east for a drop of 23 m, compared with a calculated value of 1.8 mm. Further confirmation of the earth's rotation came from the detection of stellar parallax by Calandrelli in 1806 (W.A. Wallace, 1995).
Another argument against the rotation of the earth is that objects would fly off into space if the earth were rotating, as it is well-known that objects can fly off a rotating wheel. Whether they actually do so depends on the rotational velocity; in modern terminology they fly off if the centrifugal force mv2/R due to the rotation is greater than the force holding them on the rotator, the gravitational force mg in the case of the earth. So the answer to this objection is that the earth does not rotate fast enough for this to happen. Since the period of rotation T = 2R/v, the centrifugal force is 4mh/T2 where R is the radius of the earth, T the period of its rotation and g the acceleration due to gravity. The ratio of these is gT2/4R, approximately 287. Thus the earth would have to rotate about seventeen times faster before bodies flew off.
Many of the arguments Galileo used against the Aristotelians have already been mentioned. He was able to show that many Aristotelian beliefs, especially those concerning the immutability of the heavens, are demonstrably false. The attempts of the Aristotelians to evade his arguments, by saying for example that what Galileo saw was due to defects in his telescopes, were quite unconvincing. None of this had any direct bearing on the motion of the earth, but it did expose defects in the Aristotelian system and thus weaken the belief in the centrality and immovability of the earth.
In the context of his times, the argument that a moving earth contradicted Scripture was the most serious. It could be met in two ways, depending on one's beliefs concerning the way Scripture should be interpreted. The simplest way, supported by a considerable theological tradition, is to say that Scripture does not contain any statements about the nature of the world, and if it appears to do so then it is a matter of using colloquial speech. Thus we still say that the sun rises in the morning, without committing ourselves to geocentrism. More succinctly, in the words of Cardinal Baronius, Scripture teaches us how to go to heaven, not how the heavens go.
Galileo might have been wise to keep to this argument, but in the aftermath of the Council of Trent a more rigorous interpretation was imposed, namely that the literal sense of Scripture must be strictly adhered to unless there was definite proof to the contrary. Thus heliocentrism was excluded unless it could be shown to be true. If this were done, then the theologians would be obliged to re-interpret Scripture in accord with the new cosmology. Galileo was willing to accept this stricter view because he believed that he could indeed demonstrate the correctness of heliocentrism. In this belief he was over-optimistic, and this led to his downfall.
Part of the difficulty is to know what constitutes a proof, and this was not specified by the theologians, and indeed it is difficult to see how they could have done so. Except in some very clear and obvious cases, this is a matter of some difficulty, and it depends on one's whole philosophy of science. Furthermore, the acceptability of a proof depends on the knowledge of the reader; someone ignorant of science would be unable to judge whether a proof is valid or not.
Galileo could have avoided all his troubles by saying that heliocentrism was just a calculational device that bore no relation to reality, and indeed he was strongly urged to do this. But scientists, and Galileo was no exception, know that they are investigating an objectively-existing world, and such a subterfuge is unacceptable.
Galileo was thus committed to proving the motion of the earth. Concerning its daily rotation, it is very unlikely that all the stars could move so regularly around the earth with the necessary very high velocities precisely adjusted to keep them in the same relative positions. It is dynamically so much simpler to attribute the observed appearance to the rotation of the earth. Concerning the planets, Ptolemy found that they required two circular motions to describe their positions, and one of these always had a period of exactly one year. Is this just a coincidence? Is it not much more plausible to attribute this to the yearly motion of the earth around the sun? Scientists accept such arguments as being very strong; it is not necessary to prove that there is no other conceivable explanation. And yet such considerations do not amount to a conclusive proof.
It is an ironic comment that the rotation of the earth was subsequently demonstrated by Foucault's pendulum, an experiment that in principle could have been carried out by Galileo. It is interesting to speculate how this would have been received by the Aristotelians, as its interpretation requires the concept of inertial motion. The translational motion of the earth was proved by James Bradley's discovery of stellar aberration in 1729.
Another argument depends on the apparent brightness of the planets. According to the heliocentric model, the distance of the planet Mars from the earth should vary by a factor of seven or eight, resulting in a brightness variation of about sixty, compared with the observed ratio of four or five. This was, however, a visual estimate, and Galileo was able to show that more accurate telescopic observations gave a brightness ratio in full agreement with the Copernican model.
The proof that Galileo considered the strongest was his explanation of the tides. It had long been known that the tides follow a daily cycle with high and low tides about every twelve hours, a monthly cycle due to the time-lag of about fifty minutes each day, the half-monthly cycle with high tides at new and full moon and the half-yearly cycle with higher tides at the equinoxes than at the solstices. It was also necessary to account for the very different magnitudes of the tides at different places, from the absence of tides in lakes, the small tides in the Mediterranean to the much larger tides in the oceans.
Marcantonio de Dominis had already attributed the daily cycle to the attraction of the oceans by the moon, but this was usually rejected because it would give only one tide a day when the moon is over the ocean. This argument is however mistaken because, just as the moon attracts the ocean nearest to it more than the earth, so it attracts the earth more than the ocean at the other side of the earth, giving two high tides each day. Galileo firmly rejected the possibility that the moon is responsible for the tides, considering it to be the invocation of 'occult qualities and similar idle imaginings' (Shea, 1977, p.181). Just the same objection was later to be directed against Newton when he proposed his theory of gravitation. Galileo also rejected the lunar explanation of the tides as another instance of 'explaining by naming' (as when, for instance, we explain free fall by 'gravity'). This is a valid objection until we have a quantitative theory.
Instead, Galileo sought a physical explanation of the tides and attributed the tides to the double motion (rotation and revolution) of the earth, modulated by the extent and orientation of the sea. He observes that just as the water at the edge of a basin rises and falls as the basin is moved back and forth, so do the oceans rise and fall as the earth moves. The two motions of the earth, rotation and revolution, are indeed uniform, but when combined they sometime reinforce and sometimes oppose each other, so the result is a non-uniform motion. This explanation is ingenious, but it is incorrect in principle and fails to account for the known association between the tides and the moon. Galileo had apparently forgotten his earlier argument that since the air and the oceans share the motion of the earth they can have no effect on the tides. Furthermore, even if his argument were correct, it would imply that high tides should occur at noon and low tides at midnight, which is not the case.
However, Galileo was convinced that his model was correct and he tried to explains the obvious objections by reference to the configurations of the seas, but this was not convincing. He considered the alternative explanations to be just fantasies, and he brushed them aside.
Thus, although Galileo was able to advance many very cogent arguments for the motion of the earth, the one he emphasised most strongly is fallacious, and none of them amounted to a strict proof. He thus failed to meet the over-rigorous conditions that he had accepted in order to show that heliocentrism is not inconsistent with Scripture.
The final theoretical proof of heliocentrism for physicists was provided by Newton's laws of motion and his theory of gravitation. From these, Kepler's three laws of planetary motion can be deduced and all the motions of the planets calculated to high accuracy. Thereafter there was no doubt that the heliocentric theory is correct.
In recent years it has been said that since Einstein has shown that all motion is relative the whole question of whether the earth goes round the sun or the sun around the earth is now meaningless. It is not necessary to embark on an analysis of the concept of inertial motion to show that this argument is fallacious; it is sufficient to ask what the solar system would look like to someone on a spaceship at some distance away from it.
5 The Comets
Three comets appeared in 1618, and naturally excited great interest. Galileo was urged by his friends to write about them, but he was bedridden at the time with arthritis and was unable to make any observations. Eventually he yielded to their request, particularly because it was being said that the motions of the comets provide a strong argument against the Copernican system. Among the many publications on the comets, he singled out for detailed criticism one by Fr Grassi, a professor at the Collegio Romano. Grassi published an answer to Galileo's remarks, and Galileo responded with a lengthy analysis eventually published as Il Saggiatore (The Assayer), a combination of brilliant rhetoric and erroneous science.
Galileo began with a summary of ancient theories of comets, showing that they are all unsatisfactory, except the view of Pythagoras that they are refractions of our vision of the sun, like the rainbow or the aurora borealis: "In my opinion this effect has no other origin than that a part of the vapour-laden air surrounding the earth is for some reason unusually rarefied, and being extraordinarily sublimated rises above the cone of the earth's shadow so that its upper part is struck by the sun, and made to reflect its splendour, thus causing the aurora borealis". He thought that comets move in straight lines, maintaining that this accounts for all the basic observations. There were however as Grassi showed several difficulties that he was unable to answer satisfactorily.
Galileo got himself into these difficulties because he thought that the heliocentric theory would be threatened if comets are real bodies moving along orbits. He was further inhibited by his belief in the simplicity of nature, and would only accept linear and circular orbits, neither of which is appropriate for comets. Rather than admit other orbits he preferred to deny that comets are real and reduced them to optical phenomena. He rejected the possibility of a very large orbit, apparently forgetting that this is required by the heliocentric theory. Indeed, in this discussion of comets Galileo behaved just like a conservative Aristotelian.
6 The Scientific Achievements of Galileo
The main achievement of Galileo was to inaugurate a new way of thinking about the world. He rejected the traditional method of seeking the answers to physical problems by studying the works of masters such as Aristotle and replaced it by quantitative measurement and analysis. Instead of philosophical discussions about the nature of motion he measured as accurately as possible how long it took for bodies to fall a certain distance, and then tried to find a mathematical relation between them. He was not the first to emphasise the importance of experiment; others like Robert Grosseteste in the thirteenth century had laid the foundations of experimental science. But he was the first to stress the importance of establishing mathematical relationships between the results of measurements. He was thus a pioneer in the mathematicisation of nature, which ultimately led to the science of theoretical physics.
Galileo maintained that "philosophy is written in that great book which ever lies before our eyes, I mean the universe, but we cannot understand it if we do not first learn the language and grasp the symbols in which it is written. This book is written in the mathematical language, and the symbols are triangles, circles and other geometrical figures, without whose help it is humanly impossible to comprehend a single word of it, and without which one wanders in vain though a dark labyrinth".
It is difficult for us to realise the magnitude of this achievement. Aristotle had a rather low view of mathematics, and many others, from Swineshead to Hegel, doubted its usefulness in studying physical problems. Indeed, it is not at all obvious that mathematics can be applied to the physical world: when we look around us there is so much that is apparently chaotic and unpredictable. One can also argue that mathematics does no more than provide a formal description of phenomena, whereas physics described the real processes taking place, as illustrated for example by Ptolemy's epicycles and Aristotle's physics. Why then should we expect the abstractions of mathematics to apply to material things? Even now, scientists such as Einstein and Wigner have expressed their astonishment at the 'unreasonable effectiveness of mathematics'. There have indeed been many unsuccessful attempts to apply mathematics to subjects like medicine and psychology. It was a stroke of genius for Galileo to select the one phenomenon that shows the effectiveness of mathematics most clearly, namely the free fall of bodies, and even then he had to abstract from irrelevant considerations such as the shape, colour and material of the body and the retarding effect of the medium and also overcome the difficulties of time measurement. Mathematics is now essential for physics; indeed Rontgen remarked that "the physicist in preparing for his work needs three things, mathematics, mathematics and mathematics".
It is not correct to describe Galileo's scientific method either as Platonist or as hypothetico-deductive (Wallace 1981, p.129 et seq). He believed that nature is simple and that we can attain true knowledge of it. He did not accept the view that a scientific theory just saves the appearances, enabling us to calculate results that are more or less in accord with observations and measurements but tell us nothing about the real nature of the world. He was thus a realist, and he built on and combined the methods of Archimedes and Aristotle to forge a new method, and this is one of his chief claims to be regarded as the founder of modern science.
The hypothetico-deductive method can be expressed in the form 'if p, then q; but q, therefore p'. This is fallacious because there may be other p's that also imply q. So how can we attain certainty in scientific reasoning? Galileo starts from a form of reasoning known as ex suppositione, used by Archimedes and Aristotle, and further elaborated by Aquinas. This may be expressed in the form: "if p, then 'if p then q', then q". This requires considerable explanation, and can be illustrated by the example of a lunar eclipse used by the medieval commentators. We can show geometrically that a lunar eclipse occurs when the earth is between the sun and the moon, so that it intercepts the light from the sun that would otherwise fall on the moon. This is genuine, certain, theoretical knowledge. From this we can infer that if the required conditions for an eclipse actually occur, then there will be an eclipse, unless some other effect intervenes to prevent it. It is important to notice that the first part of the reasoning is exact, theoretical and geometrical and refers to an ideal world, whereas the subsequent inference refers to the real world where our knowledge is approximate and subject to various uncertainties, due to a multitude of other effects that could conceivably interfere with what usually happens. Nevertheless, we have certainly attained genuine knowledge of the phenomenon, and have taken into account the possibility of interfering effects. As described by Archimedes, the demonstration uses formal (mathematical) causality and the relationships refer to quantifiable aspects of the subject, and any defects arise because mathematical entities are not exactly realised in the real world. For Aristotle the demonstration uses efficient causality and the defects are due to the matter involved or to the agent.
Galileo combined these modes of reasoning, which takes the following form in the case of a projectile shot horizontally. First he shows mathematically that if such a projectile is shot in a vacuum, then its motion will be a combination of a uniform horizontal motion (s=vt) and an accelerated downward motion (s=gt /2), giving a semi-parabolic trajectory. Then, if a real projectile is shot in this way, and accepting the existence of air resistance and other disturbing influences, but knowing that they are small, we deduce that the resulting trajectory will be quite closely a semi-parabola, and that the time of fall will be quite closely that given by our calculation. One again, we have genuine, true knowledge, though it is not exact. In addition to his calculations, Galileo went beyond Archimedes by making careful experiments, and verified the correctness of his results, thus showing that it is possible to apply mathematics to natural phenomena in this way, and that the disturbing influences are indeed small. The argument is no longer qualitative, as it was for Aristotle, but quantitative and indeed exact, if we understand (as did Galileo) that 'exact' means 'exact within the limitations of measurement' and in the limit when the disturbing effects are reduced to zero. In this stress on exactness Galileo differed from Plato, although Plato also stressed the importance of mathematical forms, for him these applied exactly only to an ideal world, and only imperfectly to the real world. Galileo thus developed and used the method that is implicitly used by working scientists today.
Galileo unified what had previously been separated, and separated what had previously been entangled. Through his astronomical discoveries and his application of mathematics to celestial phenomena he overcame Aristotle's sharp separation and unified terrestrial and celestial phenomena. Everything is now subject to the same laws, and these are expressible in mathematical form. This unification, Galileo recognised, is not easily attained. Initially, each realm of phenomena has to be studied with its own concepts and laws, and eventually they may be united with other phenomena, as electric and magnetic phenomena were unified by Maxwell. This work is still in progress: gravitation and quantum mechanics still await unification. Many of what are sometimes regarded as Galileo's greatest discoveries were anticipated in one way or another by scholastic philosophers and other Renaissance mathematicians, and his achievement was to unify them in a way that led to the development of theoretical physics.
In addition to his work of unification, Galileo separated science from theology, in the sense that theology, and in particular the Bible, should not be used as a source of scientific knowledge. Rather, what we find out by observation and experiment can sometimes assist the interpretation of the Bible. Galileo is sometimes credited with the elimination of metaphysics from science, but no scientific activity can ever be free from metaphysical assumptions. In particular, Galileo had a strong belief in the order and simplicity of nature and in its real existence, and these and other beliefs presupposed by science are Christian beliefs that played a determining role in the origin of modern science in the High Middle Ages. Likewise, Galileo separated physics from the principles of Aristotelian philosophy.
Although Galileo was largely responsible for the overthrow of Aristotelian physics, he retained his belief in Aristotle's natural philosophy, particularly the need to attain a physical understanding of nature and to use the laws of logic. He was a strong believer in the simplicity of nature and so continued to believe that the orbits of the planets are circular. As a result, although he greatly admired the work of Kepler, he never showed much interest in Kepler's three laws of planetary motion, which indeed provide strong support for the heliocentric theory. This belief in universal circular motion led him into great difficulties when he considered the explanation of comets. Neither did he spend time on the optical theories underlying the operation of the telescope, but constructed them by a process of trial and error.
Galileo's second great achievement was the construction of greatly improved telescopes, and the astronomical discoveries he made with their aid. These made him famous throughout Europe and changed his life. He soon became convinced that the heliocentric theory of Copernicus was correct and used his astronomical discoveries to develop arguments in its favour. Individually, none of these arguments was conclusive, and at least one was incorrect, but together they were sufficient to convince the scientifically-trained mind.
If he had been content, like most scientists, to publish his results in weighty Latin tomes, he would not have become embroiled in controversy. Instead, he vigorously publicised his work, and wrote in the vernacular in a way that could be understood by the general reader. Partly this was necessary in order to obtain employment, but in addition he felt that he had a duty to publicise his new view of the world. When his jealous enemies, unable to defeat him on scientific grounds, invoked the aid of theology, he countered with a treatise on the interpretation of Scripture. The theologians, angered by his incursion into their domain, and concerned by the threat to their whole Aristotelian world-view, used their political power in an attempt to suppress his views. They were, for the most part, well motivated, and saw themselves as defenders of a hallowed synthesis of natural and supernatural knowledge. They had, however, no understanding of Galileo's scientific achievements, or of the strength of his arguments, which made it absolutely essential for them to re-think their whole world view.
The attempt to suppress Galileo's scientific work of course failed, and his writings became widely known throughout Europe. His work on dynamics, in particular, culminated in the achievements of Newton, who unified Galileo's laws of motion and those of Kepler with his theory of gravitation. More generally, Galileo inaugurated a new style of scientific thinking that was to bear much fruit in the following centuries and identifies him as one of the founders of modern science.
Galileo and the Inquisition I
William E. Carroll
Wittgenstein once asked a friend, "Tell me, why do people always say it was natural for man to assume that the Sun went round the Earth rather than that the Earth was rotating?" His friend replied, "Well, obviously, because it just looks as though the Sun is going round the Earth." To which Wittgenstein responded, "Well, what would it have looked like if it had looked as though the Earth was rotating?"
On the occasion of the publication, in March 1987, of the Catholic Church's condemnation of in vitro fertilization, surrogate motherhood, and fetal experimentation, there appeared a cartoon in a Roman newspaper, in which two bishops are standing next to a telescope. In the distant night sky, in addition to Saturn and the Moon, there are dozens of test-tubes. One bishop turns to the other, who is in front of the telescope, and asks: "This time what should we do? Should we look or not?" The historical reference to Galileo was clear. In fact, at a press conference at the Vatican, Cardinal Ratzinger was asked whether he thought the Church's response to the new biology would not result in another "Galileo affair." The Cardinal smiled, perhaps realizing the persistent power -- at least in the popular imagination -- of the story of Galileo's encounter with the Inquisition more than three hundred and fifty years before. The Vatican office which Cardinal Ratzinger now heads, the Congregation for the Doctrine of the Faith, is the direct successor to the Holy Roman and Universal Inquisition.
In my initial lecture I sought to provide a global view of the "Galileo Affair," paying special attention to the persistence of the legend of Galileo's encounter with the Inquisition: a legend which sees Galileo as representing modern science's fighting to free itself from the clutches of blind faith, biblical literalism, and superstition. I argued that Galileo and the officials of the Inquisition shared common first principles about the nature of scientific truth and the complementarity between science and religion. In this lecture and the next two, I want to examine with you some of the particulars of this famous story in order to suggest an interpretation quite at variance with the popular legend.
Galileo was born in Pisa in 1564: the same year in which Michelangelo died and Shakespeare was born. It was 21 years after the publication of Copernicus' treatise on heliocentric astronomy; it was 47 years after the appearance of Luther's 95 theses and the beginning of the Reformation. In fact, the Protestant Reformation, the Catholic response -- especially the Council of Trent, whose final session ended in 1563 -- the destruction of the religious unity of Europe, and the ensuing wars of religion constitute the world in which Galileo will spend his entire life.
Galileo entered the University of Pisa in 1581 to prepare for a career in medicine, but his interests quickly turned to natural philosophy and mathematics. After teaching at Pisa for a few years, he left in 1592 for the University of Padua. It was at Padua, from 1592 to 1610, that he formulated the basic principles of his physics, especially his understanding of the laws of motion.
In 1609 he began to use the newly discovered telescope to observe the heavens, and in March 1610 he published The Starry Messenger in which he reported his discoveries that the Milky Way consists of innumerable stars, that the Moon has mountains, and that Jupiter has four satellites. Subsequently, he discovered the phases of Venus and spots on the surface of the Sun. He named the moons of Jupiter the "Medicean Stars" and was rewarded by Cosimo de' Medici, Grand Duke of Tuscany, with appointment as chief mathematician and philosopher at the Duke's court in Florence.
In order to understand the importance of Galileo's Starry Messenger, we need to place his observations in the context of developments in astronomy in the late sixteenth and early seventeenth centuries. Between 1572 and 1610 there were several new observations: the nova of 1572; Tycho Brahe's observations (and those of others, as well) of comets in 1577 and 1585; the super-nova of 1604; and Galileo's own observations in 1609/10. These observations persuaded several natural philosophers that some important features of the heavens described by Aristotle could no longer be accepted as accurate: in particular, the immutability of the heavens and the existence of solid, crystalline planetary spheres.
Also, as I noted in my first lecture, Galileo did not think that his telescopic discoveries provided a proof for the view that the earth rotated on its axis and revolved about the Sun. He did think that they provided arguments for the plausibility of Copernican astronomy. His discovery of the phases of Venus required only that Venus must revolve about the Sun. Even the discovery of spots on the Sun, and the fact that these spots moved across the Sun's surface, only provided evidence that the Sun was not an immutable body. None of Galileo's telescopic discoveries required the abandonment of a modified geocentric system; much less did they affirm the truth of a heliocentric one. Furthermore, Galileo understood the difference between providing plausible arguments for a position and demonstrating that it is true. Although Galileo's telescopic observations were not sufficient to demonstrate the truth of Copernican astronomy, they did serve to call into question the received geocentric cosmology, which was a melange of views having their source in Ptolemy and Aristotle. They were also a powerful incentive for Galileo to discover a demonstration for the motion of the earth. In The Starry Messenger, Galileo claimed that his most important discovery were the four moons of Jupiter. This discovery, according to Galileo,
. . . [provides] an excellent and splendid argument for taking away the scruples of those who, while tolerating with equanimity the revolution of the planets around the Sun in the Copernican system, are so disturbed by the attendance of one Moon around the Earth while the two together complete the annual orb around the Sun that they conclude that this constitution of the universe must be overthrown as impossible. For here we have not only one planet revolving around another while both run through a great circle around the Sun: but our vision offers us four stars wandering around Jupiter like the Moon around the Earth while all together with Jupiter traverse a great circle around the Sun in the space of twelve years. [Sidereus Nuncius . . . , pp. 84-5]
Copernican astronomy required two centers of heavenly motion: the Moon's revolving around the Earth, and the Earth and the other planets' revolving around the Sun. Yet, such a universe, with more than one center of motion, seemed inconceivable. Since it was now clear that four moons revolved around Jupiter, and Jupiter itself moved around another center, an important objection to Copernican astronomy disappeared.
How could any thinking person not accept the new heavens, the novità celesti, to which Galileo had drawn attention? Stillman Drake, one of the famous scholars of Galileo in our own age, categorizes the opposition to Galileo in this way:
The arguments brought forth against [Galileo's] new discoveries were so silly that it is hard for the modern mind to take them seriously. . . . The chief argument was that the phenomena he had described were merely illusions created by his telescope and had no real existence in the skies. . . . One of his opponents who admitted that the surface of the moon looked quite rugged, maintained that it was actually quite smooth and spherical as Aristotle had said, reconciling the two ideas by saying that the moon was covered with a smooth and transparent material through which [the rugged surface] could be discerned. . . . One after another, all attempts to cleanse the heavens of new celestial bodies came to grief. Philosophers had come up against a set of facts which their theories were unable to explain. The more persistent and determined adversaries of Galileo had to give up arguing and resort to threats. [Drake, pp. 73-4]
Were the objections to Galileo's claims all so silly, as Drake calls them? Galileo argued that his new optical device revealed things in the heavens as they really were, even though they were invisible to the naked eye. Galileo provided no theoretical arguments, in the science of optics, to demonstrate the reliability of the instrument he had used. In this respect, it is useful to cite the remarks of the philosopher and historian of science, Hans Blumenberg, who warns us against an all-too-easy dismissal of Galileo's opponents: "The fool's role that Galileo's opponents have long played in the historiography of natural science has rendered them harmless for us and obscured their significance as indicators of the difficulties in our relation to reality that are always present and become especially acute in historical situations where radical change is under way. The failure of their obstructed faculty of vision is only a correlate of the exaggerated expectations that Galileo himself had invested in his optical discoveries." [Blumenberg, p. 662]
In an intriguing recent article, Roger Ariew notes that Galileo's arguments for there being mountains on the Moon presupposes that the Moon reflects light from the Sun. But, to assume that the moon reflects light is to assume that the surface of the Moon is rough and uneven: to assume, that is, that the Moon is like the Earth. In this regard it is interesting to note that Medieval natural philosophers rejected the view that the Moon reflected light because a smooth and polished surface (which they were convinced the Moon was) would function like a mirror and reflect light rays in a way other than from a rough surface (i.e., the whole surface would not reflect light equally). Thus, they thought that the Moon receives light from the Sun, becomes luminescent, and then light emanates from it. Averroes is a principal source of this view:
It has been demonstrated that if the moon acquires the power of lighting up from the sun, it is not by reflection. . . . If it illuminates, it is by becoming a luminous body itself. The sun renders it luminescent first and then the light emanates from it in the same way that it emanates from the other stars; that is, an infinite multitude of rays are issued from each point of the moon. If its power of illumination issued from reflection, it would illuminate some determined places on earth depending upon its circumstances; reflection is produced only for some determined angles. . . . Since the various parts of the celestial body are distinguished with respect to whether they are translucent or not, or luminescent, it is not impossible that the various parts of the moon receive the light of the sun differently. [cited in Ariew, p. 219]
There is a long discussion in the first section of Galileo's Dialogue Concerning the Two Chief World Systems in defense of the claim, based on telescopic observations, that the surface of the Moon is rough and uneven. The interlocutors compare the reflection of light from a wall with a rough surface with that from a flat mirror, and observe that the former illuminates the entirety of the surface opposite it, whereas the reflection from the flat mirror only illuminates a small portion where its bright reflection fell. Next they examine the reflection cast by a spherical mirror and decide that only a minute area of its surface would appear illuminated to the observer. "The rest would remain . . . unilluminated and therefore invisible. . . . [T]he whole Moon would be invisible [if it were a perfectly smooth spherical surface reflecting light] since that particle which gave the reflection would be lost by reason of its smallness and great distance." [Galileo, Dialogue . . ., p. 74] In The Assayer, published a decade earlier, in response to the question as to why the Moon is not smooth, Galileo writes: "it and all the other planets are inherently dark and shine by light from the sun. Hence they must have rough surfaces (fu necessario che fussero di superficie scabrosa), for if they were smooth as mirrors (liscia e tersa come uno specchio) no reflection would reach us from them and they would be quite invisible (tutto invisibili) to us." [Drake (ed.), Discoveries and Opinions of Galileo, p. 263.]
As Ariew observes, "the conclusion one ought to draw is that, since the medieval theory of the moon is not that light is reflected off the surface of the moon, but that the moon receives sunlight in proportion to its density, Galileo's observations of mountains on the moon, which assumes that sunlight is reflected off the moon, cannot succeed in destroying the medieval lunar theory; it can only be an independent account of the moon and lunar light based on radically different premises. Galileo concludes that the moon is like the earth by claiming to see spots on the lunar surface as the shadows that mountains would cause, if the light of the moon were reflected off the surface of the moon. But since the light of the moon diffuses throughout, since one does not see a simple image of the sun reflected off the moon, to assume that the light of the moon is received by reflection would be to assume that the surface of the moon is rough -- to assume the moon is like the earth. From the perspective of medieval lunar theory, Galileo's reasoning is oddly circular." [Ariew, p. 223]
The public position which Galileo occupied in Florence from 1610 involved him in controversy. As the best-known advocate for Copernican astronomy, he was a lightning rod for criticism. Philosophers, for example, were concerned with the apparent violation of the principles of Aristotelian physics involved in the notion that the Earth moved or that celestial bodies were in any way like the Earth. Criticism also came from some theologians who were troubled about the relationship between Copernican astronomy and the Bible.
In early 1615, well after the debate had begun, a Carmelite priest in Naples, Paolo Foscarini, published an essay in which he claimed that the Bible could be interpreted in such a way as to be consistent with Copernican astronomy. Foscarini, calling upon exegetical principles of well-known Catholic theologians such as Melchior Cano, observed that "when Sacred Scripture attributes something to God or to any creature which would be improper and incommensurate, then it should be interpreted and explained . . . either metaphorically. . . or according to our mode of consideration, apprehension, conception, understanding, [and] knowing . . . [as] the Holy Spirit frequently and deliberately adopts the vulgar and common way of speaking." A classic example of this mode of speaking in Scripture are statements about God's stretching out His hand, walking in the garden, or showing emotions. All such statements must be taken metaphorically or as accommodating our limited mode of understanding God. If the claim that the Earth moves were true -- a claim about one of God's creatures -- "it would be easy," Foscarini writes, "to reconcile it with those passages of Sacred Scripture which are contrary to it . . . by saying that in those places Scripture speaks according to our mode of understanding, and according to appearances, and in respect to us. For thus it is that these bodies appear to be related to us and are described by the common and vulgar mode of thinking; namely, the Earth seems to stand still and to be immobile, and the sun seems to revolve around it." Foscarini concludes that "in matters which pertain to the natural sciences and which are discovered and are open to investigation by human reason, Sacred Scripture ought not to be interpreted otherwise than according to what human reason itself establishes from natural experience and according to what is clear from innumerable data. . . . [If the heliocentric system is true] we ought not to affirm emphatically that the sacred writings favor the Ptolemaic or the Aristotelian opinion, and thus create a crisis for the inviolable and most August sacred writings themselves. Rather we ought to interpret those writings in such a way as to make clear to all that their truth is in no way contrary to the arguments and experiences of the human sciences." [cited in Blackwell, pp. 226, 232, and 259]
Foscarini sent his essay to Cardinal Roberto Bellarmino, the learned Jesuit and important officer of the Inquisition in Rome. Bellarmino, already an old man, had spent his professional career refuting the views of Protestant theologians. Late in the 16th century he had been named Professor of Controversial Theology at the new Jesuit university in Rome, and he was skilled in the intricacies of biblical interpretation as well as in challenges to the authority of the Church.
Cardinal Bellarmino's response to Foscarini, a copy of which the Cardinal sent to Galileo, is one of the most important documents for our analysis. In April 1615, the Cardinal writes:
First . . . it appears to me that [you] and Signore Galileo are proceeding prudently by limiting yourselves to speaking hypothetically and not absolutely [ex suppositione e non assolutamente], as I have always believed Copernicus did [come io ho sempre creduto che habbia parlato Copernico]. For to say that, by assuming [che supposto] the earth moves and the sun stands still, one saves all the appearances [si salvano tutte le apparenze] better than by postulating [porre] eccentrics and epicycles is to speak well [benissimo detto]. This has no danger in it, and it suffices for mathematicians. But to wish to affirm that the sun is really fixed in the center of the heavens [che realmente il sole sta nel centro del mundo] and merely turns upon itself without traveling from east to west, and that the earth . . . revolves very swiftly around the sun, is a very dangerous thing [cosa molta pericolosa], likely not only to irritate all the scholastic theologians and philosophers, but also to harm our Holy Faith by rendering Holy Scripture false [di nuocere alla Santa Fede con rendere false le Sante Scritture]. . . .
Notice the distinction Cardinal Bellarmino draws between speaking "suppositionally" and speaking "absolutely." To speak suppositionally (hypothetically), in the sense the Cardinal means, is "to save the appearances," and in astronomy "to save the appearances" is to provide a consistent mathematical description of the observed phenomena. Hence, Bellarmino refers to the eccentrics and epicycles of Ptolemaic astronomy, which are mathematical constructs to describe observed movements in the heavens. To speak "absolutely" would be to specify what the movements in the heavens really are. This is a standard distinction employed by medieval scientists and philosophers. Aquinas, for example, observes that Ptolemaic astronomy provides only a model for the observed phenomena and that one could very well have a mathematical model in which the earth moves.
Bellarmino is wrong in thinking that Copernicus was only interested in saving the phenomena. Perhaps he is only offering pastoral advice to Galileo and Foscarini, suggesting to them a safe way to advance their arguments.
Cardinal Bellarmino next raises a theological objection:
Second. I say that, as you know, the Council [of Trent] would prohibit expounding the Scriptures contrary to the common agreement [ il commune consenso] of the Holy Fathers; and if Your Reverence would read not only all their works but the commentaries of modern writers on Genesis, Psalms, Ecclesiastes, and Joshua, you would find that all agree in expounding literally [ad literam] that the sun is in the heavens and travels swiftly around the earth, while the earth is far from the heavens and remains motionless in the center of the world [sta nel centro del mondo, immobile]. Now consider, with your sense of prudence [con la sua prudenza], whether the Church could support [possa sopportare] giving Scripture a meaning contrary to the Holy Fathers and to all the Greek and Latin expositors. Nor may one reply that this is a not a matter of faith, because if it is not a matter of faith with regard to the subject matter [ex parte obiecti], it is with regard to the one who has spoken [ex parte dicentis]. Thus that man would be just as much a heretic who denied that Abraham had two sons and Jacob twelve, as one who denied the virgin birth of Christ, for both are declared by the Holy Ghost through the mouths of the prophets and apostles.
The Cardinal's reference to the decree of the fourth session (1546) of the Council of Trent is particularly important. In addition to making clear what books constituted the canon of Scripture, the Council decreed that with respect to "matters of faith and morals" no one is permitted to interpret the Bible contrary to "that sense which Holy Mother Church, to whom it belongs to judge their true sense and meaning, has held and does hold." Nor may one interpret Scripture contrary to the "unanimous agreement" of the Church Fathers. Bellarmino extends the sense of "faith and morals" to include historical and scientific claims found in the Bible, since to deny the truth of what the Bible says on any matter calls into question the affirmation that the entire Bible is God's revealed word.
Despite the Cardinal's claim that the Church's understanding of the Bible was involved in the dispute, he is willing to examine the arguments of the new astronomy.
Third . . ., if there were a true demonstration [ci fusse vera dimostrazione] that the sun is in the center of the universe [nel centro del mondo] . . . and that the sun does not circle the earth but the earth circles the sun, then one would have to proceed with great care in explaining the Scriptures that appear contrary [che paiono contrarie], and say rather that we do not understand them than that what is demonstrated is false. But I will not believe that there is such a demonstration until it is shown to me [Ma non crederó che ci sia tal dimostrazione, fin che non mi sia mostrata]. Nor is it the same to demonstrate that by supposing the sun to be at the center and the earth in the heaven one can save the appearances, and to demonstrate that in truth [che in verità] the sun is at the center and the earth in heaven; for I believe the first demonstration may be available, but I have very grave doubts [grandissimo dubbio] about the second, and in the case of doubt one must not abandon [non si de[v]e lasciare] the Holy Scripture as interpreted by the Holy Fathers. . . .
This final paragraph in Bellarmino's response to Foscarini is very important. Note, that he again draws a distinction between saving the appearances and demonstrating the truth of a position. Note further that, despite his very grave doubts, he admits the possibility of a demonstration for the motion of the earth, although he is aware of no such demonstration. In the absence of such a demonstration, prudence, at least, requires that the traditional interpretation of those passages of the Bible which claim that the earth is motionless, be maintained.
Galileo shared Cardinal Bellarmino's understanding of the difference between an astronomy which "saves the appearances" and an astronomy which demonstrates what is truly so. In a note to a friend in 1615, Galileo observed: "Two kinds of suppositions have been made . . . by astronomers: some are primary and with regard to the absolute truth in nature; others are secondary, and these are posited imaginatively to render an account of the appearances in the movements of the stars . . . ." These latter suppositions, designed to save the appearances, are, according to Galileo, "chimerical and fictive . . . false in nature, and introduced only for the sake of astronomical computation." Galileo described his task as the discovery of the "true constitution of the universe," an understanding which is "unique, true, real, and which cannot be other than it is." [Galileo, Opere, Vol. 5, 102]
Galileo the scientist shares with Aristotle and Aquinas, and with Cardinal Bellarmino, the view that science deals with the truth of things. It is important to remember that the Aristotelian notion of science that was current in the age of Galileo is different from what we generally consider science today. Scientific knowledge for Aristotle is knowledge of what is necessarily so, that is, cannot be otherwise, because it is based on the discovery of the causes that make things be what they are. Such sure, certain knowledge is quite different from the product of probable or conjectural reasoning: reasoning which lacks certitude because it falls short of identifying true and proper causes. Galileo, despite his disagreements with 17th century Aristotelians, never departed from Aristotle's ideal of science as sure, certain knowledge. Whether Galileo was arguing about the movement of the earth or about laws that govern the motion of falling bodies, his goal was to achieve true, scientific demonstrations. Cardinal Bellarmino exemplifies the same Aristotelian position: namely, that the natural scientist discovers the truths of nature. Thus, he demands that if Galileo, the scientist, wishes to speak "absolutely," he must provide a demonstration for the motion of the Earth: after all, that is what a good scientist does. Without a demonstration a scientist cannot conclude that, in fact, the Earth moves. Although Cardinal Bellarmino accepted the Aristotelian notion of science, he was more than ready to reject specific conclusions in Aristotelian cosmology. When he was a young professor at Louvain in the 1570s he embraced a biblical cosmology at odds with many of the details of Aristotle's description of the heavens. In particular, Bellarmino rejected Aristotle's view that the heavens were immutable and composed of special matter. In the 1570s and again in the second decade of the 17th century, Bellarmino admits that, were the confusion in astronomical theories resolved by scientific truth, "one would have to consider a way of interpreting the Scriptures which would put them in agreement with the ascertained truth, for it is certain that the true meaning of Scripture cannot be in contrast with any other truth, philosophical or astronomical." [Baldini and Coyne, p. 20]
The opposition within scientific circles in the early 17th century to claims that the Earth moved was generally based on the assumption that a geocentric astronomy was an essential part of a larger Aristotelian cosmology: the view, that is, that Aristotelian physics and metaphysics depended in some way on the affirmation that the Earth was immobile at the center of the universe. Thus, if one were to reject such a geocentric astronomy, then, so it seemed to many, the whole of Aristotelian science would have to be discarded. As a result of such an understanding of the interdependence of astronomy, cosmology, physics, and metaphysics, the acceptance of a moving Earth would involve a radical philosophical revolution. Hence, we might understand why many of Galileo's contemporaries were so troubled by his support for Copernican astronomy. Furthermore, although we now accept without question that the Earth moves, we need to guard against assuming that it is a simple matter to reach this conclusion and that, therefore, the scientific opponents of Galileo were either simple-minded or stubbornly blind to the truth.
An understanding of the theological dimensions of the encounter between Galileo and the Inquisition requires that we keep in mind this question concerning the scientific knowledge of the motion of the Earth. All sides in the controversy were committed to the Aristotelian ideal of scientific knowledge. Remember, Cardinal Bellarmino told Galileo that if there were a demonstration for the motion of the Earth, then the Bible would have to be interpreted accordingly. The Cardinal has simply reaffirmed traditional Catholic teaching that the truths of science and the truths of faith cannot contradict one another. Whether we turn to Augustine in the 4th century or Aquinas in the 13th, we can discover the common Catholic commitment to the harmony between reason and revelation. Furthermore, both Augustine and Aquinas warned against using the Bible as an encyclopedia of natural science. Galileo liked to quote the remarks of Cardinal Baronius: Scripture teaches you how to go to heaven, not how the heavens go.
In the next lecture, we will look at Galileo's response to Cardinal Bellarmino's letter to Foscarini and examine how what was prudential advice, that is, to avoid speaking as though Copernican astronomy were true, becomes in 1616 a disciplinary order of the Inquisition, according to which Galileo is required not to hold, teach, or defend the view that the Sun is in the center of the universe and that the Earth moves.
Texts Cited
Ariew, Roger. "Galileo's Lunar Observations in the Context of Medieval Lunar Theory," Studies in the History and Philosophy of Science 15, no. 3 (1984), pp. 212-227.
Baldini, Ugo and G. V. Coyne. The Louvain Lectures of Bellarmine and the Autograph Copy of his 1616 Declaration to Galileo. Vatican Observatory Publications, 1984.
Blackwell, Richard J. Galileo, Bellarmine, and the Bible. University of Notre Dame Press, 1991.
Blumenberg, Hans The Genesis of the Copernican World (trans. by Robert Wallace). Cambridge, Mass.: MIT Press, 1987.
Drake, Stillman (ed.) Discoveries and Opinions of Galileo. Garden City, New York: Doubleday, 1957.
Van Helden, Albert (trans./ed.). Sidereus Nuncius or The Sidereal Messenger. Chicago: The University of Chicago Press, 1989.
Galileo and the Inquisition II
William E. Carroll
"The Sun's going down. Or the Earth's coming up, as the fashionable theory has it. -- Not that it makes any difference." Rosencrantz in Rosencrantz and Guildenstern are Dead by Tom Stoppard
Galileo's telescopic observations, described in The Starry Messenger and the Letters on Sunspots, provided the occasion for a renewed consideration of Copernican astronomy. At the very least, certain conclusions of traditional geocentric astronomy could no longer be maintained. Christopher Clavius, the famous Jesuit mathematician at the Collegio Romano, had written as early as 1611 that as a result of what Galileo had discovered and what he himself had seen through the telescope, astronomers will now have "to consider how the celestial orbs may be arranged in order to save these phenomena."
As we have seen, an understanding of the encounter between Galileo and the Inquisition requires that we keep in mind the question of the scientific status of the claim that the Earth revolves about the Sun. I have already indicated that Galileo and Cardinal Bellarmino understood science in the Aristotelian sense, as sure and certain knowledge of what is so: knowledge, the certainty of which is guaranteed by a causal nexus. As the discussion of Copernican astronomy moved increasingly into the arena of theology -- and, in particular, into the realm of what the Bible was said to reveal on the subject -- Galileo was concerned that the Church might be persuaded (or persuade itself) to condemn the new astronomy.
We know that, by 1615, Galileo was convinced that he was on the verge of achieving a demonstration for the motion of the Earth, but he needed time. He sought to prevent the Church from making a foolish mistake: condemning as heretical the claim that the Earth moves, when he was about to demonstrate that in fact the Earth does move. Galileo expected that an argument from the phenomenon of the tides would provide the necessary demonstration. He circulated a manuscript on this subject in late 1615 and early 1616, and the argument appears in the final section of his Dialogue Concerning the Two Chief World Systems, published in 1632. But in 1615 and 1616, Galileo did not think that he yet had the requisite demonstration. There is some debate among Galileo scholars as to whether he eventually thought that he was able to demonstrate the motion of the Earth from the fact of the ocean tides; I think that Galileo came reluctantly to the conclusion, by the 1620's, that he didn't have such a demonstration, although he found the argument ingenious and included it in the Dialogue.
In any event, in 1615 and 1616 neither Galileo nor the Inquisition thought there was a demonstration for the motion of the Earth: Galileo expected, indeed anticipated, one; the Inquisition did not. In the absence of a demonstration for the motion of the Earth, Cardinal Bellarmino had urged prudence: do not challenge the traditionally accepted readings of those biblical passages which have been interpreted as affirming the immobility of the Earth. The cardinal was acutely aware of the Protestant challenges to the Catholic Church's claim to be the sole, legitimate interpreter of God's word. In many ways we see the Inquisition especially concerned with maintaining the authority of the Church against all who seemed to threaten it. Bellarmino, veteran of theological controversies with Protestants, was always alert to point out that the principal theological issue of the day with respect to the Bible was not so much what Scripture meant as who had the authority authentically to interpret it. For the cardinal "the protection of the Church's interpretive authority was a separate and more basic issue than the specific question of whether the motion of the Sun and the stability of the Earth should be taken as simply literal or figurative. . . . The individual judge of Scripture (e.g., Galileo or Foscarini) faced a double jeopardy; one relating to the content of the interpretation, the other to assuming the role of interpreter." [Blackwell, pp. 36-7] No matter what the merits of the interpretation offered, the individual was always subject to the suspicion that he was usurping the Church's divinely established role as the guardian of God's word.
During the momentous years of 1615 and 1616, as the discussion about the relationship between the new astronomy and the Bible reached into the highest circles of the Catholic Church, Galileo was increasingly concerned at the authorities in the Church might act foolishly and conclude that there was an incompatibility between heliocentric astronomy and the Bible.
We can see the general outline of Galileo's position in the notes he wrote to himself as he sketched his response to Bellarmino's letter to Foscarini:
The motion of the earth and the stability of the sun could never be against Faith or Holy Scripture, if this proposition were correctly proved to be physically true by philosophers, astronomers, and mathematicians, with the help of sense experience, accurate observations, and necessary demonstrations. However, in this case, if some passages of Scripture were to sound contrary, we would have to say that this is due to the weakness of our mind, which is unable to grasp the true meaning of Scripture in this particular case. This is the common doctrine, and it is entirely right, since one truth cannot contradict another truth. On the other hand, whoever wants to condemn it judicially must first demonstrate it to be physically false by collecting the reasons against it. . . . If the earth de facto moves, we cannot change nature and arrange for it not to move. But we can rather easily remove the opposition [la repugnanza] of Scripture with the mere admission that we do not grasp its true meaning [il suo vero senso]. Therefore the way to be sure not to err is to begin with astronomical and physical investigations, and not with scriptural ones. [Finocchiaro, pp. 80-2]
Galileo addresses the question of the relationship between science and the Bible in his most extensive and systematic way in his famous "Letter to the Grand Duchess Christina." Galileo is the chief scientist in the employ of the Medici family and Christina of Lorraine is the mother of the reigning Grand Duke. The letter contains Galileo's account of the recent controversy over the claims of Copernican astronomy. He composes it in 1615, after having read Bellarmino's response to Foscarini, and, as I mentioned, in the midst of the debate concerning the relationship between traditional interpretations of the Bible and the view that the Earth moves.
By addressing the letter to the Grand Duchess, rather than to theologians in Rome, Galileo is able to write to an educated lay audience, even though his primary audience are the authorities of the Inquisition in Rome. Galileo is neither a bishop nor a theologian, and theologians in Rome might well dismiss a theological treatise addressed to them by Galileo the mathematician and physicist.
Galileo is well-trained in Renaissance techniques of rhetoric and a failure to recognize Galileo's rhetorical techniques has resulted in uncritical reading of the letter. For example, many modern history texts accept without question Galileo's own account of the history of the controversy, which he presents in the first few paragraphs of the letter. We must remember when we read his account that, first of all it is his interpretation of the events, and, second, that he has chosen his facts carefully in order to achieve his end: to persuade the authorities of the Catholic Church not to act foolishly and condemn Copernican astronomy.
He identifies his enemies as being unable to refute him in science, and as a result, they "try to shield the fallacies of their arguments with the cloak of simulated religiousness and with the authority of Holy Scripture, unintelligently using the latter [the Bible] for the confutation of arguments they neither understand nor have heard." The story he tells of Copernicus is also interesting. He misidentifies him as a priest, argues that his investigations were undertaken at the request of the Pope, and, noting that Copernicus' book was dedicated to the Pope, Galileo claims: "Once printed this book was accepted by the Holy Church, and it was read and studied all over the world without anyone's ever having the least scruple about its doctrine." Galileo concludes his historical observations with the following remark:
Finally, now that one is discovering how well founded upon clear observations and necessary demonstrations [quanto ella sia ben fondata sopra manifeste esperienze e necessarie dimostrazioni] this doctrine is, some persons come along who, without having seen the book, give its author the reward of so much work by trying to have him declared a heretic; this they do only in order to satisfy their special animosity, groundlessly conceived against someone else [Galileo, himself] who has no greater connection with Copernicus than the endorsement of his doctrine.
Note what Galileo claims and what he does not claim. His comments, at first glance, suggest that Copernican astronomy has been demonstrated to be true, or perhaps has been shown to be true on the basis of "clear observations" [manifeste esperienze], no doubt Galileo's telescopic discoveries. But on closer inspection, we see that all Galileo is claiming is that Copernican astronomy is "well founded upon clear observations and necessary demonstrations." To show that a position is "well founded" is not necessarily to show that it has been demonstrated to be true. Galileo is aware of the importance of necessary demonstrations; he has in mind Bellarmino's distinctions in the Cardinal's letter to Foscarini. In fact, throughout the "Letter to the Grand Duchess," Galileo uses the phrase "necessary demonstrations" frequently, without once offering such a demonstration for the motion of the earth. Remember the rhetorical nature of the Letter; Galileo seeks to persuade the officers of the Inquisition not to condemn Copernican astronomy. Galileo knows that theologians in Rome accept the position that the truths of science and the truths of faith cannot contradict one another, and that, if there is a scientific demonstration on a particular subject, it would not be possible for the Bible to be authentically interpreted in a way which contradicts what science demonstrates. Remember, in addition, that both Galileo and the officers of the Inquisition share the same Aristotelian ideal of scientific knowledge; both sides understand what a demonstration is. If Galileo, in fact, had a demonstration for the motion of the earth, he surely would have presented it, for he knew that a demonstration would prevent the Church's condemnation of Copernican astronomy. We see here another reason for ostensibly addressing the letter to the Grand Duchess, for she would not be expected to follow a complex scientific demonstration; it would be sufficient for her chief scientist simply to suggest that one existed. Throughout the "Letter to the Grand Duchess," Galileo reaffirms traditional Catholic teaching on the relationship between science and scripture. God is the author of both the book of nature and the book of scripture. Therefore, the truths of nature and scripture cannot contradict one another. [verum cum vero congruit] Accordingly, Galileo writes:
I think that in disputes about natural phenomena one must begin not with the authority of scriptural passages, but with sensory experiences and necessary demonstrations. For the Holy Scripture and nature derive equally from the Godhead, the former as the dictation of the Holy Spirit and the latter as the obedient executrix of God's orders; moreover, to accommodate the understanding of the common people it is appropriate for Scripture to say many things that are different in appearance and in regard to the surface meaning of the words from the absolute truth . . . and so it seems that natural phenomena which are placed before our eyes by sensory experience or proved by necessary demonstrations should not be called into question, let alone condemned, on account of scriptural passages whose words appear to have a different meaning.
One must know how to discover the true meaning of Scripture since this does not always correspond to the surface meaning of the words (il nudo significato delle parole). Furthermore, we must remember that the primary purpose of the Bible is not to reveal how the heavens go, but how to go to heaven. In his attempt to persuade the Inquisition not to condemn Copernican astronomy, Galileo asks rhetorically: "can an opinion be both heretical and irrelevant to the salvation of souls?" Cardinal Bellarmino might well respond that there are different senses of "irrelevant." It may well be that, from the point of view of the subject matter, an astronomical claim is irrelevant to salvation, but if this issue is discussed in the Bible then the question is relevant to salvation in that it is a matter of faith that the Bible is true from beginning to end. We have seen, however, that Cardinal Bellarmino understands that the true meaning of the Bible may be difficult to discern in a particular instance; the Cardinal is well aware that the true meaning of the text may be expressed in metaphors and similes. For example, when the Bible refers to God's stretching out His hand, the true meaning of the text concerns God's power since God does not have a body.
Galileo does recognize the authority of the Church to determine the true meaning of the Bible, but he urges those in Rome to beware of the mischievous advice and that the Church should not "flash the sword [simply because she] . . . has the power to do it, without considering that it is not always right to do all that one can do." Galileo argues that it is contrary to Catholic practice "to use scriptural passages to establish conclusions about nature, when by means of observation and necessary demonstrations one could at some point demonstrate the contrary of what the surface meaning of the words affirm."
Galileo quotes famous Catholic theologians, most notably, St. Augustine, and he leaves these quotations in the authoritative, original Latin. He finds support for the continuity of his views with Catholic orthodoxy in passages from Augustine's On the Literal Meaning of Genesis such as this one: "In obscure subjects very far removed from our eyes, it may happen that even in the divine writings we read things that can be interpreted in different ways by different people, all consistent with the faith we have; in such a case, let us not rush into any one of these interpretations with such precipitous commitment that we are ruined if it is rightly undermined by a more diligent and truthful investigation." [I.18]
Another passage from Augustine serves Galileo's purposes well: "There should be no doubt about the following: whenever experts of this world can truly demonstrate something about natural phenomena, we should show it not to be contrary to our Scripture; but whenever in their books they teach something contrary to the Holy Writ, we should without any doubt hold it to be most false and also show this by any means we can. . . ." [On the Literal Meaning of Genesis I.21] After citing this text from Augustine, Galileo employs a particularly deft argument: the words of St. Augustine imply
. . . the following doctrine: in the learned books of worldly authors are contained some propositions about nature which are truly demonstrated and others which are simply taught; in regard to the former [those truly demonstrated], the task of wise theologians is to show that they are not contrary to Holy Scripture; as for the latter (which are taught but not demonstrated with necessity), if they contain anything contrary to the Holy Writ, then they should be considered indubitably false and must be demonstrated such by every possible means.
In which of these two categories would one put the argument for the motion of the Earth in 1615? Galileo is so certain that he is about to have a demonstration for the motion of the Earth that he grants to the Bible an authority on scientific matters that both Augustine and Aquinas would deny. Perhaps he thought that such obeisance to biblical authority on his part might ingratiate him to the Inquisition in Rome. In fact, in a rather clever move, Galileo seems to turn the tables on the Inquisition: "therefore, before condemning a physical proposition, one must show that it is not conclusively demonstrated. Furthermore, it is much more reasonable and natural that this be done not by those who hold it to be true, but by those who regard it to be false . . . ."
You might imagine how a theologian in Rome would evaluate Galileo's argument. Despite all the rhetoric of necessary demonstrations, one searches the letter in vain to find one. Rather than providing a scientific demonstration, Galileo expects theologians to enter the arena of science to show that a particular proposition is not "conclusively demonstrated."
In the eighth and final lecture, on Galileo as theologian, I will return to the arguments Galileo set forth in this letter and will examine the widely accepted view that he embraces a modern distinction between science and religion. For now I wish to ask, in the light of so many shared views about the nature of scientific demonstration, the absence of such a demonstration for the motion of the Earth, the ancient Catholic commitment that the truths of science and the truths of faith cannot contradict one another, how it is that what was for Cardinal Bellarmino prudential advice, to consider Copernican astronomy purely as an hypothesis, was transformed in 1616 to an explicit injunction to Galileo not to hold, teach, or discuss Copernican astronomy?
In early 1616 the cardinals of the Inquisition instruct their theological consultants formally to consider the status of the new astronomy in the light of biblical revelation. In February 1616 the consultants reach the conclusion that the propositions of Copernican astronomy concerning the mobility of the Earth and the immobility of the Sun are false and heretical. We will return to examine again the conclusions of the theological experts in the final lecture on Galileo as theologian, but for now we should note that a report of theological consultants does not constitute Church doctrine. Furthermore, the theologians first conclude that the claims that the Sun is immobile and the Earth moves are false scientifically, and then they conclude that these propositions are heretical in that they contradict the literal sense of the Bible. As I mentioned in my initial lecture, the theologians do not conclude that these scientific claims are false because they contradict the Bible: that is, they do not subordinate scientific claims to biblical authority. Rather, they come to understand what they consider to be the true sense of the Bible after they have concluded that the new astronomy is false on scientific grounds. On the basis of these findings, the Inquisition orders Galileo not to hold, teach, or defend such propositions, and the text of Copernicus must no longer be published until it is corrected. The corrections eventually ordered by the Index of Forbidden Books involve changing those passages in which Copernicus claims that in fact the Earth moves to read that he simply supposes or hypothesizes that the Earth moves. The order for the correction of Copernicus' text is instructive: "If certain of Copernicus' passages on the motion of the Earth are not hypothetical, make them hypothetical; then they will not be against either the truth or the holy writ. On the contrary, in a certain sense, they will be in agreement with them, on account of the false nature of suppositions, which the study of astronomy is accustomed to use as its special right." The distinction between speaking hypothetically and speaking absolutely, which Bellarmine had urged upon Galileo in April 1615, as prudential advice, now serves as the basis for the disciplinary decrees of the Inquisition and the Index of Forbidden Books. The theologians of the Inquisition, committed as they were to the complementarity between science and scripture, accepted as obviously true a particular geocentric cosmology, and, on the basis of such a commitment, insisted that the Bible must be read in a certain way. Furthermore, just as some philosophers mistakenly concluded that Aristotelian physics and metaphysics depended on a geocentric cosmology, so some theologians feared that, a rejection of Aristotle's view that the Earth does not move, would call into question all of Aristotelian philosophy, a philosophy upon which important elements of Catholic theology depended. Catholic theologians, for example, had long employed Aristotelian physics in their exposition of the doctrine of transubstantiation. The role of Eucharistic theology in the Galileo Affair has been the subject of considerable interest since the publication of Pietro Redondi's book, Galileo eretico, in 1983. Redondi's thesis concerns theological objections to atomistic physics found in Galileo's Assayer, which was published in 1623. In some respects, Redondi is reiterating a claim made by Thomas Campanella in 1622: "The first argument against Galileo is that it seems that theological doctrines would be completely overthrown by anyone who tries to introduce new ideas which are contrary to the physics and metaphysics of Aristotle, on which St. Thomas and all the Scholastics based their theological writings." [A Defense of Galileo (trans. by Richard Blackwell, U. of Notre Dame Pres, 1994), p. 43] We will consider Redondi's thesis in some detail in the next lecture.
What is clear in the actions of 1616 is that the theologians of the Inquisition thought that the Bible contained scientific truths. Since it was obvious, from science, that the Earth does not move, and since certain passages in the Bible seemed clearly to say or to imply the same thing, it must be the case that the Bible proclaims that the Earth does not move. Furthermore, in the face of the Protestant Reformation, the Catholic Church was particularly alert to threats, real or imagined, to traditional interpretations of the Bible and to the authority of the Church to determine the true meaning of the Bible.
The Inquisition did not think that it was requiring Galileo to choose between faith and science. Nor, in the absence of scientific knowledge for the motion of the Earth, would Galileo have thought that he was asked to make such a choice. One week after learning of the 1616 decision of the Inquisition, Galileo wrote to the Secretary of State to the Grand Duke of Tuscany to inform him as to what had transpired. Galileo had been in Rome since December 1615, hoping to use his influence to prevent the condemnation of Copernican astronomy. He was acutely aware that some Florentine theologians had accused him of affirming a position that was "heretical and against the faith." Galileo observed that his opponents "tried orally and in writing to make this idea prevail, but events have shown that [this] . . . effort did not find approval with the Holy Church. She [the Church] has only decided that the opinion does not agree with Holy Scripture, and thus only those books are prohibited which have explicitly maintained that it does not conflict with Scripture." [6 March 1616] Although Galileo did not mention in this letter the specific injunction communicated to him, it seems that he did not view what transpired to be as serious as it has since been interpreted. In May 1616 Galileo obtained a formal statement from Cardinal Bellarmino concerning what had happened when the Cardinal had informed Galileo of the Inquisition's orders to him in March of that year. According to Bellarmino, Galileo had not been required to recant any views he held; "he [Galileo] has only been notified of the declaration made by the Holy Father and published by the Sacred Congregation of the Index, whose content is that the doctrine attributed to Copernicus (that the earth moves around the sun and the sun stands at the center of the world without moving from east to west) is contrary to Holy Scripture and therefore cannot be defended or held." [Finocchiaro, p. 153]
The famous trial of Galileo in 1633, after the publication of his Dialogue Concerning the Two Chief World System, depends on the decisions reached 17 years earlier. The theological, philosophical, and scientific questions which constitute the heart of the controversy are clear by 1616. The Inquisition expected Galileo to obey their orders not to hold, teach, or defend Copernican astronomy. The cardinals who sat in judgment of Galileo in 1633 were convinced that he had violated that injunction and they demanded that he formally renounce the views proscribed seventeen years before.
Texts Cited
Blackwell, Richard J. Galileo, Bellarmine, and the Bible. University of Notre Dame Press, 1991.
Fantoli, Annibale. Galileo: for Copernicanism and for the Church. (translated by George Coyne), second edition. Vatican Observatory Publications, 1996.
Finocchiaro, Maurice A. (ed.) The Galileo Affair: A Documentary History. The University of California Press, 1989.
Galileo and the Inquisition III
William E. Carroll
Galileo had been committed to publishing the Dialogue Concerning the Two Chief World Systems for some time. Already in The Starry Messenger in 1610 he promised to provide a demonstration for the motion of the Earth in a future book on the "system of the world." During the second decade of the 17th Century, Galileo was occupied with replying to, criticizing, and refuting his critics on a number of fronts: before, during, and after his visit to Rome (29 March to 4 June 1611) concerning his telescopic observations; in his debates with opponents in Tuscany concerning his views on floating bodies (1611-1615); in arguments with Chirstopher Scheiner over sunspots (1613); concerning theological objections to Copernican astronomy (1612-1616); and with Orazio Grassi on the nature of comets. Galileo also spends time working out details of his discoveries, such as calculating the periods of revolution of the moons of Jupiter. As we have seen, in March of 1616 the Inquisition orders Galileo not to defend Copernican astronomy.
I have already mentioned that in late 1615 he had circulated a treatise in which he argued that the phenomenon of the ocean tides might provide the kind of evidence which would lead necessarily to the conclusion of the double motion of the Earth as the cause. It is this argument which appears in the fourth day of the Dialogue. The 1616 injunction of the Inquisition did not prevent the private circulation of Galileo's treatise on the tides but it did mean that Galileo would have to adjust his plans to write a book on the system of the world, which would have been simply an elaboration of his argument for Copernican astronomy on the basis of the tides.
In April 1624, the year after the election of Cardinal Maffeo Barberini as Pope Urban VIII, Galileo journeyed to Rome and had six long audiences with his old friend. Galileo had dedicated The Assayer to the newly elected Pope in 1623, and the Pope had listened approvingly as the book was read to him. Reports of their discussions indicate that the Pope told Galileo that there would be no problem in discussing Copernican astronomy so long as he restricted his presentation to the hypothetical. We have already encountered one sense of "hypothetical" in the traditional view that mathematical astronomy was hypothetical in that it simply "saved the appearances;" it could not in principle arrive at the truths of heavenly motions. The distinction between "hypothetical" and "true," and the confusion concerning different senses of "hypothetical," play an important role in Galileo's encounter with the Inquisition. In Bellarmino's letter to Foscarini (1615), which we have already discussed, the cardinal drew a distinction between mathematical astronomers who speak hypothetically (ex suppositione) and "save the appearances" by using epicycles, deferents, and the like, and physicists who have as their goal the discovery of the true structure of the cosmos. Bellarmino, following Thomas Aquinas, was well aware that Ptolemaic astronomy fell into the former category. Epicycles and eccentrics are geometric devices to describe observed celestial motions and as such were "hypotheses." Geometric entities could not serve as necessary, physical causes of the observed motions of the heavens. Bellarmino noted that, since such hypotheses could not, in principle, constitute a true science of the heavens, there was no danger of their being in conflict with biblical truths. As we have seen, Bellarmino did not deny that there can be scientific knowledge about the heavens; he did not think, however, that mathematical astronomy is such a science.
There was considerable ambiguity in the use of the term "hypothetical" in the early seventeenth century. Thus, when different interlocutors (Galileo, Bellarmino, Pope Urban VIII, officers of the Inquisition) use this term they do not always mean the same thing. In addition to the Thomistic tradition, in which Bellarmino participated, there was another one according to which the "hypothetical" status of claims about nature reflected a scepticism about human intelligence: the tendency, that is, to view any claim about human knowledge as "hypothetical." This emphasis on the ephemeral, uncertain character of knowledge of the world was . . . reinforced by the voluntaristic theology and nominalistic philosophy of the fourteenth century." [Feldhay] Thus, to affirm divine omnipotence required, so it seemed, the denial of the possibility of human knowledge of the world. Since science, in the traditional Aristotelian sense, was knowledge of a necessary nexus between cause and effect, it seemed that to argue that this type of knowledge is possible was to necessitate God, and hence to deny divine omnipotence. Cardinal Agostino Oregio (1577-1635), a colleague and friend of Maffeo Barberini (later Pope Urban VIII), reports a conversation Barberini had with Galileo sometime in 1615 or 1616. Barberini defended the view that, given God's omnipotence and omniscience, we ought not to "bind divine power and wisdom" by claiming that any human science knows for sure the way things are. Oregio was one of the theological consultants engaged by the Inquisition in 1632 to examine Galileo's Dialogue. He also is a source for the conversations Pope Urban VIII had with Galileo in 1624, in which the Pope asked Galileo whether he agreed that unless you can show a particular claim about nature contains a contradiction then you have to admit that God has "the power and wisdom to arrange differently [from any theory Galileo advances] the orbs and the stars in such a way as to save the phenomena that appear in heaven." If God has the power and wisdom to arrange the heavens in a way different from any theory which we propose , while saving all the phenomena, "then we must not bind divine power and wisdom" by saying that a particular explanation of the heavens is true." The pope confided to another of his cardinals that there ought to be no fear about Copernican astronomy since no one could possibly demonstrate it to be necessarily true. It seems that the pope understood the "hypothetical" character of Copernican astronomy to mean that it cannot possibly be true. This is a different understanding of "hypothetical" from that which Aquinas used when he wrote of hypotheses in mathematical astronomy: as devices for saving the appearances they could not be necessarily true since they did not conform to the principles of a true science of nature.
"Everyone agreed that it was permissible to use . . . [Copernican astronomy] as a hypothesis. The epistemological status of a hypothesis, however, was related to the major scientific, philosophical, and theological debates of the period. The interpretation of the concept 'hypothetical knowledge' thus became dependent on the theological and philosophical orientation of the interpreter." [Feldhay] One recent scholar [Feldhay] has drawn attention to the disputes between the Jesuits and the Dominicans concerning grace and free will, which resulted in a tendency toward intellectual skepticism among the Dominicans and "changed the status of new forms of knowledge like Copernicanism from that of unproven doctrines to that of unprovable ones." In the discussions surrounding the publication of the Dialogue, the Pope's chief theologian, Niccolo Riccardi, wrote to the inquisitors in Florence that the title of the book could not be On the Ebb and Flow of the Tides, and that the opening and closing of the book had to reaffirm the hypothetical nature of the discussion. Riccardi's letter reveals the continuing importance for the Pope of maintaining the hypothetical character of the arguments concerning Copernican astronomy: the Pope thinks, Riccardi wrote, "that the title and subject should not focus on the ebb and flow but . . . on the mathematical examination of the Copernican position on the Earth's motion, with the aim of proving that, if we remove divine revelation and sacred doctrine, the appearances could be saved with this supposition. . . ." Galileo's negotiations with Riccardi in the early summer of 1630 resulted in the inclusion of an introduction, "to the discerning reader," in which Galileo claimed that he wrote the book to show that the 1616 prohibition of books that argued for the truth of the new astronomy was not the result of scientific ignorance in Catholic circles. His book would show that Catholic thinkers knew the arguments, but rejected them, and he would do this, as he says in his preface, by taking "the Copernican point of view, proceeding in the manner of a pure mathematical hypothesis and striving in every contrived way to present it as superior to the viewpoint of the Earth's being motionless."
By writing the book in the form of a dialogue among a proponent of Copernicus (Salviati), a supporter of the Ptolemaic and Aristotelian positions (Simplicio), and an intelligent, uncommitted third party (Sagredo), Galileo thought that he would preserve the claim that he, the author, did not hold or teach Copernican astronomy. The arguments Galileo advanced for Copernican astronomy in the text did not claim to be a demonstration for the new astronomy, but nor did they take the form of mere fictive models to save the phenomena. In the first three parts (days) Galileo shows that none of the arguments advanced for the immobility of the Earth and the mobility of the Sun are conclusive; in fact, the defender of geostatic astronomy, Simplicio, does not fare well in the dialogue with Sagredo and Salviati. The demolition of arguments against the Earth's motion leads to the discussion in the fourth part of the positive argument for such motion based on the tides. Galileo does follow Riccardi's instructions to emphasize the suppositional character of the arguments in the final part of the book. It is, however, Simplicio, the defender of the discredited geostatic position who remarks:
I confess that your idea [concerning the tides] seems to me much more ingenious than any others I have heard, but I do not thereby regard it as true and conclusive. Indeed, I always keep before my mind's eye a very firm doctrine, which I once learned from a man of great knowledge and eminence, and before which one must give pause. From it I know what you would answer if both of you [Sagredo and Salviati] are asked whether God with His infinite power and wisdom could give to the element of water the back and forth motion we see in it by some means other than by moving the containing basin; I say you will answer that He would have the power and the knowledge to do this in many ways, some of them even inconceivable by our intellect. Thus, I immediately conclude that in view of this it would be excessively bold if someone should want to limit and compel divine power and wisdom to a particular fancy of his.
Galileo certainly thought that scientific knowledge of nature was possible; he did not accept the view that appeals to divine omnipotence rendered all human claims to knowledge "hypothetical." When authorities in Rome read the book they were convinced that Galileo had defended, in some way, Copernican astronomy. He had done precisely what in 1616 he had been enjoined not to do.
In late summer of 1632 the Pope ordered that publication of the book cease, and he appointed a special commission to examine it. In September 1632 the papal commission concluded that Galileo ìmay have overstepped his instructions by asserting absolutely the Earth's motion and the Sun's immobility and thus deviating from hypothesis. . . . The commission noted that when Galileo brought his original manuscript to Rome in 1630 it was clear that although "he had been ordered to discuss the Copernican system only as a pure mathematical hypothesis, one found immediately that the book was not like this, but that it spoke absolutely, presenting the reasons for and against without deciding." [Finocchiaro, p. 220] Thus, Riccardi had insisted on a preface and an ending in which Galileo makes it explicit that he is only going to write hypothetically and that the entire text should conform to this approach. One of the conclusions commission reached was that, despite the changes in the beginning and end of the book, Galileo had not really followed Riccardi's instructions. The commission also discovered the 1616 document according to which the Inquisition instructed Galileo not to "hold, teach, or defend it [Copernican astronomy] in any way whatever, orally or in writing." The key phrase, "in any way whatever," plays an important role in the Inquisition's judgment of Galileo's guilt.
One of the theologians asked to review the book for the Inquisition in 1633, Melchior Inchofer, concludes that throughout the book Galileo proceeds in a "categorical, absolute, and nonhypothetical manner." No doubt, Inchofer is employing the term "hypothetical" in that sense which opposes it to speaking "absolutely." As Inchofer observed: "Galileo promises [in the book] to proceed in the manner of a mathematical hypothesis, but a mathematical hypothesis is not established by physical and necessary conclusions. . . . So Galileo should have posited the Earth's motion as something to be analyzed deductively, not as something to be proved true by destroying the opposite view, as he indeed does in the entire work. . . . So, in order to restrict himself to a pure mathematical hypothesis, Galileo did not have to prove absolutely that the Earth moves, but only to conceive its motion in the imagination without assuming it physically, and thereby explain celestial phenomena and derive the numerical details of the various motions." [Finocchiaro, p. 267]
Thus, even though Galileo might say that he did not claim to demonstrate that the Earth moves, he still was not speaking "hypothetically" in the sense that the authorities in Rome required. In the formal sentence of June 1633, the Inquisition noted that the Dialogue explicitly violated the 1616 injunction since Galileo, in this book, ìdefended the said opinion [of the Earth's motion and the Sunís stability] already condemned and so declared to your face, although in the said book you try by means of various subterfuges to give the impression of leaving it undecided and labeled as probable; this is still a very serious error [errore gravissimo] since there is no way an opinion declared and defined contrary to divine Scripture may be probable [non potendo in niun modo esser probabile un'opinione dichiarata e difinita per contraria alla Scrittura divina] [Finocchiaro, p. 289] Note the argument that one cannot say that an opinion is probable if it has been declared and defined to be contrary to the Bible. It is important to remember the distinction between possible and probable. Probable means that the preponderance of evidence favors a view. Obviously, a Catholic must use the evidence of what Scripture says in determining whether a position is probable. To defend the opinion that the Earth moves and the Sun stands still as "probable" would mean that one had ignored or seriously undervalued the clear evidence of the Bible. The certificate Galileo had from Cardinal Bellarmino (May 1616), which he presented at the Inquisition's proceedings in 1633, did not contain the injunction that he should not teach, hold, or defend, orally or in writing, in any manner whatsoever, Copernican astronomy. This certificate, however, did attest to the fact that Galileo had been told that this opinion was contrary to Scripture: a fact which only aggravated Galileo's case further in the eyes of his judges, since it shows that Galileo knew that the new astronomy was contrary to Scripture and yet he "dared to treat of it, defend it, and show it as probable."
After preliminary discussions before the Inquisition in April 1633, Galileo admitted that his book offered a stronger support for Copernican astronomy than he really intended, and that he regretted the error and would make changes in the book. A summary of the proceedings was sent to the pope, who decided that Galileo must be interrogated to see what his intentions were in writing the book. This interrogation occurred on 21 June and Galileo denied any malicious intent. The cardinals of the Inquisition who sat in judgment of Galileo concluded that he was "vehemently suspected of heresy," a formal category less serious than being guilty of heresy, which would have included willful perseverance in a false doctrine. Galileo was required to make a public abjuration on the 22nd of June in which he acknowledged that it was heretical to hold the view that the Earth moves and the Sun stand s still, and that, accordingly, he rejected such a view.
As we have seen, the trial of Galileo before the Inquisition in 1633, and his official abjuration, depend on the events of 1616. Pietro Redondi, whom I mentioned before, thinks, howcver, that the trial was a kind of theatre of shadows orchestrated by Pope Urban VIII to protect his friend Galileo, and himself, from far more serious charges of Eucharistic heresy. Although the specific claim of a behind-the- scenes conspiracy on the part of the Pope lacks any evidentiary foundation, Redondi's analysis raises important questions about the relationship between theology and science in the early 17th century.
According to Redondi, the offending book is not the Dialogue Concerning the Two Chief World System, published in 1632, but The Assayer, dedicated to Pope Urban VIII, and published in 1623. The ostensible subject of The Assayer is a discussion of the nature of comets. It contains an eloquent account of Galileo's philosophy of science. The oft-quoted passage that the Book of Nature is written in the language of mathematics is found in this book, and the text is a tour de force in scientific methodology. Here is how Redondi categorizes The Assayer:
The rejection of dogmatic submission to the principle of authority in the field of philosophy; the vindication of a new language; the rights of research and free intellectual discourse against the prevarication of institutional culture -- these were the contents that made The Assayer the manifesto of the new philosophy in Rome. The book was a literary sensation because, even more than the Jesuits, even more than Scholastic thought, it seemed to oppose a whole intellectual tradition. The telescope was the instrument through which one looked at the entire universe, and The Assayer was the manual that taught one to read the universe like a book.
Redondi discovered in the secret archives of the Inquisition a new document, denouncing arguments in The Assayer, a document which he thinks is the key to unlock the secrets of the Galileo affair. The document is a denunciation to the Inquisition of certain passages in the book in which Galileo argues for atomistic physics. The author of the denunciation points out the problems of such a physics for the doctrine of the real presence of Christ in the Eucharist: a doctrine recently reaffirmed in the Council of Trent's definition of transubstantiation. That doctrine required, so it seemed, an acceptance of the truth of Aristotle's distinction between substance and accident. If one were an atomist, that is, if one thought that reality is exhaustively understood in terms of indivisible particles and their movement in space, how could one also believe that the real "accidents" of the bread and wine (color, taste, and the like) remained the same while the substance of the bread and wine became the body and blood of Christ? As the author of the denunciation remarks:
In the [consecrated] host, it is commonly affirmed, the sensible species (heat, taste, and so on) persist. Galileo, on the contrary, says that heat and taste, outside of him who perceives them, and hence also in the host, are simple names; that is, they are nothing. One must therefore infer, from what Galileo says, that heat and taste do not subsist in the host. The soul experiences horror at the very thought.
In the offending passage from The Assayer, Galileo addresses the question of whether heat is a real phenomenon, a property or quality of an object. Ultimately, he concludes that heat is the name we give to a reaction in us; it is not a quality of an external object. His argument is worth citing in some detail:
[W]henever I conceive any material or coproreal substance, I immediately feel the need to think of it as bounded, and as having this or that shape; as being large or small in relation to other things, and in some specific place at any given time; as being in motion or at rest; as touching or not touching some other body; and as being one in number, or few, or many. From these conditions I cannot separate such a substance by any stretch of my imagination. But that it may be white or red, bitter or sweet, noisy or silent, and of sweet or foul odor, my mind does not feel compelled to bring in as necessary accompaniments. Without the senses as our guides, reason or imagination unaided would probably never arrive at qualities like these. Hence I think that tastes, odors, colors, and so on are no more than mere names so far as the objects in which we place them is concerned, and that they reside only in the consciousness. Hence if the living creature were removed, all these qualities would be wiped away and annihilated. . . . To excite in us tastes, odors, and sounds I believe that nothing is required in external bodies except shapes, numbers, and slow or rapid movements. . . . Those materials which produce heat in us and make us feel warmth, which are known by the general name of 'fire,' would then be a multitude of minute particles having certain shapes and moving with certain velocities. Meeting with our bodies, they penetrate by means of their extreme subtlety, and their touch as felt by us when they pass through our substance is the sensation we call 'heat.' [Drake, pp. 274 and 277]
The Catholic doctrine of the Eucharist requires that one believe that "the whole substance of the bread" and the "whole substance of the wine" change into the body and blood of Christ, with "only the appearances of bread and wine remaining." [Canon 2, Thirteenth Session of the Council of Trent, 1551] If the distinction between the substance of the bread and the taste, color, and the like of the bread is denied, how can one adhere to the Council of Trent's definition of transubstantiation? Yet, it is precisely such a distinction between substance and accidents which an atomistic physics denies: thus, the threat of such a physics for Eucharistic theology.
The doctrine of transubstantiation required a substance be separated even from its extension (i.e., its quantity) such that a new substance (i.e., Christ) could come into being while the accidental forms of bread and wine, the extension, color, taste, etc. were miraculously sustained in the separated quantity of the bread and wine. The content of faith -- the real presence of Christ -- was made intelligible by using categories of Aristotelian philosophy. The threat of atomism, according to Redondi, is that there is no distinction, in fact there cannot be a distinction, between the quantitative extension of a thing, however, small, and the substantial reality of the thing. Thus, an atom of bread is not separable into substance and its various accidents. If the extended body of bread existed then no change has taken place. Transubstantiation, which affirms precisely such a change, is thus unintelligible! "The soul experiences horror at the very thought."
Redondi thinks that "beneath the words 'heat', 'smell', and 'taste', lay centuries of Eucharistic debate which had again become topical. [Neither Galileo nor the theologians] could be unaware that Aristotelian philosophy was wedded to scholastic theology through the dominant interpretation of these words. . . . The Assayer proposed a materialistic theory of sensible phenomena to sever the knot: to separate natural philosophy from scholastic theology."
As we have seen, according to Redondi, The Assayer served as a manifesto for a new culture which, in the eyes of the defenders of Tridentine Catholicism, had to be resisted. The Galileo Affair, thus, must be seen as part of a battle between an old guard and the new regime of Urban VIII. This is the context in which, according to Redondi, the Pope seeks to deflect the charges of eucharistic heresy by ordering a trial based on Galileo's defense of Copernican astronomy. Redondi is right in emphasizing the importance of understanding the controversy between Galileo and the Inquisition in its historical context, and he provides an excellent description of the cultural climate of the early 17th century. He reminds us again and again of the nexus of theology, the natural sciences, literature, and the arts.
Although Redondi's reconstruction of the cultural world of the Galileo Affair is compelling, that reconstruction serves as a Procrustean bed into which he forces the evidence of the trial, or, better, simply ignores the evidence of what he calls a "theatre of shadows." Redondi persuades himself that his thesis must be true because he is unable to accept that the theologians of the Inquisition could take seriously the problem of the relationship between the Bible and Copernican astronomy. I hope that I have been able to show you how it is that the theologians could, and indeed did, take this question seriously.
How then do we understand the "Galileo Affair"? Despite the powerful legend of the warfare between science and theology, we need to recognize that the errors in judgment committed by the theologians of the Inquisition involved the subordination of the interpretation of certain biblical passages to a particular cosmology, and that these errors resulted in disciplinary abuses, not doctrinal falsehoods. Without a demonstration for the motion of the Earth, it was indeed possible to believe that the Bible affirmed that the Earth did not move. To insist upon such an affirmation, however, is to violate principles established by Augustine and Aquinas. Nevertheless, the controversy between Galileo and the Inquisition is inconceivable were it not the case that both sides shared common principles: the complementarity between faith and reason, the Bible and science; the role of the Church as the authentic interpreter of scripture; and a commitment to an Aristotelian ideal of demonstration in science In an ironic sense, we might say that the "Galileo Affair" offers ample testimony, not for the warfare between science and theology, but for the harmony between the two.
Texts Cited
Drake, Stillman (ed.) Discoveries and Opinions of Galileo. Garden City, New York: Doubleday, 1957.
Finocchiaro, Maurice A. (ed.) The Galileo Affair: A Documentary History. The University of California Press, 1989.
Feldhay, Rivka. Galileo and the Church: Political Inquisition or Critical Dialogue? Cambridge University Press, 1995.
Redondi, Pietro. Galileo Heretic. (trans. by R. Rosenthal). Princeton University Press, 1987 [Galileo eretico, Einaudi, 1983].
Galileo: Theologian
William E. Carroll
In the front of his own copy of the Dialogue Concerning the Two Chief World Systems, Galileo wrote the following:
Take care, theologians, that in wishing to make matters of faith of the propositions attendant on the motion and stillness of the Sun and the Earth, in time you probably risk the danger of condemning for heresy those who assert the Earth stands firm and the sun moves; in time, I say, when sensately or necessarily it will be demonstrated [col tempo, dico, quando sensatamente o necessariamente si fusse dimostrato] that the Earth moves and the Sun stands still.
This passage reveals many of themes crucial for understanding the controversy between Galileo and the Inquisition. We find in it both Galileo's commitment to demonstrations in science and his admission that there is not yet such a demonstration for the motion of the Earth. The passage also reaffirms a key principle Galileo set forth in the "Letter to the Grand Duchess Christina": that when investigating physical questions one should not begin with biblical texts. Galileo warns the theologians to avoid acting imprudently, lest they be faced with the unpleasant task of condemning as heretical those propositions which they now declare to be orthodox.
Such diverse commentators on Galileo as Giorgio di Santillana, author of The Crime of Galileo, and Pope John Paul II have praised the astuteness of Galileo's theological observations on the relationship between science and scripture. In ceremonies commemorating the 100th anniversary of the birth of Einstein in 1979, and again in October 1992, the Pope, referring to the fundamental compatibility between science and the Bible, quoted approvingly from Galileo's "Letter to the Grand Duchess Christina," in which Galileo observed that God is author of all truth, both the truth of nature and the truth of Scripture. di Santillana's praise of Galileo's theology is effusive: "[Galileo] warns and exhorts with the dignity of a [Church] Father of the early centuries...." and "The content of his spurned and incriminated theological letters has become official Church doctrine." di Santillana laments that there was no "young Aquinas" in Rome in 1616 to follow Galileo's lead in theology. A more circumspect scholar such as Owen Gingerich, professor of the history of astronomy at Harvard, writes recently that the Catholic Church should now accept "Galileo's arguments about the reconciliation of science and scripture." Gingerich acknowledges that the theological principles enunciated by Galileo have "long since" been adopted by Protestant and Catholic theologians, but he still thinks it would be useful for the Catholic Church to make an official pronouncement confirming Galileo's theological arguments.
Even in the recent work of Mauro Pesce of the University of Bologna, who has written extensively on Galileo's principles of biblical exegesis, we find the claim that Galileo represents a missed opportunity for the Church in the seventeenth century to discover an adequate relationship between modernity and religion. According to Pesce, it was not until Pope Leo XIII's encyclical, Providentissimus Deus, in 1893, that the Church would accept, even in an attenuated form, the principles enunciated by Galileo. Thus, between February 1616, when the theologians of the Inquisition condemned Copernican astronomy, until 1893, there was a continual official refusal to accept Galileo's proposals for the compatibility between religion and science. According to Pesce, the fundamental issue from 1616 to 1893 was not really the acceptance of Copernican astronomy, but rather the unwillingness to accept the hermeneutical principle that the truth of Scripture is religious and not scientific. Pesce claims that it was this distinction between science and religion which constituted the core of Galileo's claims in his letters between 1613 and 1615, and, furthermore, that it was the rejection of this principle which lies behind the condemnation of Copernicus. Galileo, in such a view, has become an icon for modern culture, and thus this final lecture returns to the themes I set forth in my initial lecture on the legend of Galileo.
In this lecture I want to examine in some detail Galileo's theological arguments concerning the relationship between science and Scripture, as they are found in a series of four letters and related notes he writes from 1613 to 1615: letters to Benedetto Castelli, Piero Dini, and the Grand Duchess Christina of Tuscany. Galileo writes these letters in response to an increasing campaign waged by academic and theological opponents: priests and professors who were convinced that Copernican astronomy and its apparent implications for Aristotelian physics, cosmology, and metaphysics, presented a serious threat to the traditional interpretation of the Bible, as well as to the whole edifice of Catholic theology. Despite these wider implications for the relationship between Aristotelian thought and Christian faith, the debate which raged in the second decade of the 17th century had as its focus the Bible and the compatibility of the new astronomy with certain passages in the Bible. The debate took on a special urgency in that, in the previous century, the Council of Trent, in response to the challenges of Protestant thinkers, had forbidden Catholics to interpret the Bible contrary to the sense of the sacred text which "the Church . . . has held and holds . . . or contrary to the unanimous teachings of the Fathers. . . ." As I pointed out in a previous lecture, Cardinal Roberto Bellarmino reminded Galileo of precisely this point when he wrote to him in April 1615: "And if [you] . . . . would read not only their works [the works of the Church Fathers] but the commentaries of modern writers . . . you would find that all agree in expounding literally that the sun is in the heavens and remains motionless in the center of the world. Now consider in all prudence," Bellarmino continues, "whether the Church could support the giving to Scripture of a sense contrary to the holy Fathers and all the Greek and Latin expositors."
The Cardinal was well aware that the injunction of the Council of Trent referred to the interpretation of Scripture concerning matters of faith and morals. But Bellarmino reminded Galileo that one could not simply say that the new astronomy was not a matter of faith and morals and thus exempt from the strictures of the decree of Trent: for even if not a matter of faith with respect to its subject [ex parte obiecti], it was, he noted, a matter of faith in that God is the unerring author of all of Scripture [ex parte dicentis], including those passages which describe astronomical phenomena. Thus one could not deny what texts from Psalms and the Book of Joshua said about the immobility of the earth and the mobility of the sun because in doing so one would challenge the divine authorship of the Bible.
Nevertheless, Bellarmino was willing to reject the traditional reading of the Bible "if there were a true demonstration that the sun was in the center of the universe .... and that the sun did not go around the earth but the earth went around the sun." If there were such a scientific demonstration, Bellarmino admitted that it would then "be necessary to use careful consideration in explaining the Scriptures that seemed contrary, and we should rather have to say that we do not understand them than to say that something is false which has been proven."
Galileo was well aware of the concerns enunciated by Cardinal Bellarmino. Nearly two years before, in a brief letter to Benedetto Castelli, his protege and professor of mathematics at the University of Pisa, Galileo sketched in outline what would become the full-blown treatise ostensibly addressed to Christina, Grand Duchess of Tuscany. Since 1610, when Galileo first published the results of his astronomical observations and began his long, public defense of Copernican astronomy he had to counter the arguments of those who appealed to the Bible to defend traditional geocentric cosmology. Galileo was chief mathematician and philosopher in the Medici court in Florence and in the letter to the Grand Duchess, penned after Bellarmino sent his letter in April 1615, we find Galileo's response to the arguments advanced by his opponents. We have already examined the basic elements of the letter, but it is worth reiterating the two general principles Galileo sets forth:
1. There can be no contradiction between the truths of science and the truths of faith. God is the author of all truth: both the truth known through revelation and the truth known through reason alone. This is hardly a revolutionary position for a Catholic to maintain. Augustine and Aquinas admit as much, as did Cardinal Bellarmino. Remember, the Cardinal observed that were there to be a demonstration that the Earth moved, then the Church could not maintain that the Bible revealed the opposite. Indeed, Cardinal Bellarmino and Galileo shared the same Aristotelian understanding of what a demonstration in science is. Science for them was necessary knowledge in terms of causes.
2. The purpose of God's revelation in Scripture is not to teach men about natural philosophy but to lead them to salvation. In the words of Cardinal Baronius, quoted approvingly by Galileo: "the Bible teaches us how to go to heaven, not how the heavens go." Although Galileo does emphasize more than do his contemporaries the distinction between the essentially religious purpose of the Bible and other truths which it may contain, he does not really anticipate a radical separation between religious and other truths in the Bible. As we examine this question more fully, we will see that there is less of a difference between Galileo and the theologians of the Inquisition than is generally thought.
When Gingerich, di Santillana, and the Pope refer to Galileo's insights on the relationship between science and Scripture, these are the two principles to which they refer. What so many see as particularly modern in Galileo's understanding of the relationship between the Bible and science is but the reaffirmation of traditional Catholic thinking, easily seen in the writings of Augustine and Aquinas. Mauro Pesce claims, in addition, that Galileo goes further than either Augustine or Aquinas in that he makes an epistemological claim in distinguishing science from religion (the Book of Nature is read more easily than the Book of Scripture), and, furthermore, that Galileo, at least implicitly, lays the groundwork for a modern conception of religion. Pesce, as well as many others, see the letter to the Grand Duchess as one of the charter documents of the modern world: a call for the emancipation of scientific reflection from the forces of traditional religion and ecclesiastical authority.
Galileo, however, was not content to reaffirm traditional Catholic principles. He sought to use these principles to urge the Church not to condemn Copernican astronomy. Yet, it is important to recognize that the principles he employed were shared by the theologians of the Inquisition.
Even if one admits that the purpose of the Bible is not to teach scientific truths, still the question remains: does the Bible have any authority at all in evaluating questions in the natural sciences? To grant that the purpose of the Bible is to lead human beings to salvation does not mean that the Bible has nothing to say about the world of nature. Galileo address this subject in the following way. He affirms that the Bible cannot err, but quickly adds that the inerrancy of the Bible concerns its true meaning [il suo vero sentimento] and not what what "its bare words" may signify [che suona il puro significato delle parole]. A slavish adherence to the "unadorned grammatical meaning" [nel nudo suono literale] of any particular passage may lead to follies, error, and heresy. One may come to think, for example, that God has hands, feet, eyes, that He gets angry and is subject to other emotions. The Bible often contains passages written in a mode "to accommodate" these passages to "the capacities of the common people” [per accomodarsi all capacita del vulgo].
Too many translators of these texts miss an important distinction. When Galileo refers to "il nudo" or "il puro" "significato delle parole," "il nudo suono litterale," or similar phrases, he does not mean the literal sense of scripture. As Aquinas and others had observed, the literal sense of the Bible, which is always true, is what the Author, ultimately God, intends the words to mean. Galileo, observing this same distinction between what we might call a literal and a literalistic reading of the Bible, distinguishes between a naive literalism and "il vero sentimento" (the true meaning) of the text. The literal sense is not the same as what the bare words signify. Galileo, thus, is embracing, not challenging, a traditional Catholic principle of biblical exegesis.
Cardinal Bellarmino was well aware of the difficulties in discovering the truths in Scripture. Every sentence in the Bible has a literal or historical meaning, i.e., "the meaning which the words immediately present." The literal meaning is either simple, "which consists of the proper meaning of the words," or figurative, "in which words are transferred from their natural signification to another." When the Bible refers to "the right hand of God," the simple literal sense would mean a part of God's body; whereas the figurative literal sense means God's power. Since, if we were to read the passage according to the simple literal sense there would be an absurd attribution of a body to God, a figurative sense must be the true meaning of this passage. There are as many different types of figurative meaning as there are types of literary figures, but all these figurative meanings are the literal sense of Scripture.
On the basis of such distinctions -- and notice that the examples Galileo uses concern passages in the Bible which attribute certain human attributes to God, and with which obviously Bellarmino would agree -- Galileo, with rhetorical deftness advances a wider argument:
Hence I think that I may reasonably conclude that whenever the Bible has occasion to speak of any physical conclusion [alcune conclusione naturale] (especially those which are very abstruse and hard to understand), the rule has been observed of avoiding confusion in the minds of the common people which would render them contumacious toward the higher mysteries. . . . Who, then, would positively declare that this principle [of accommodation] has been set aside, and the Bible has confined itself rigorously to the bare and restricted sense of its words [i puri ristretti significati delle parole], when speaking but casually of the earth, of water, of the sun, or of any other created thing? Especially in view of the fact that these things in no way concern the primary purpose of the sacred writings, which is the service of God and the salvation of souls. . . .
This being granted, I think that in discussions of physical problems [problemi naturali] we ought to begin not from the authority of scriptural passages [non si dovrebbe cominciare dalle autorità di luoghi scritture], but from sense experience and necessary demonstrations [ma alle sensato esperienze e dalle dimostrate necessarie]; for the Holy Bible and the phenomena of nature proceed alike from the divine Word, the former as the dictate of the Holy Ghost and the latter as the obedient executrix of God's commands. It is necessary for the Bible, in order to be accommodated to the understanding of every man [per accomodarsi all'intendimento dell'universale], to speak many things which appear to differ from the absolute truth [dal vero assoluto] so far as the bare meaning of the words [al nudo significato delle parole] is concerned.
If we compare these passages in the letter to the Grand Duchess with the same or similar passages in the 1613 letter to Castelli, we discover some interesting differences. Although many passages from the 1613 letter appear verbatim in the 1615 letter to the Grand Duchess -- still the 1615 letter is greatly expanded. In 1613, Galileo writes to Castelli using almost the same words he will employ in 1615, save for the observation that: "in physical disputes [disputi naturali] it [the Bible] should be reserved to the last place [ella doverebbe esser riserbata nell'ultimo luogho]." In 1615, in the passage quoted above, Galileo argues that "we ought not to begin from the authority of scriptural passages." This change -- from reserving the Bible to last place in discussing scientific questions to the admonition not to begin from the authority of scripture -- is indicative of the rhetorical thrust of the Letter to the Grand Duchess. For the real audience Galileo addresses is not the Grand Duchess, but theologians and Church officials in Rome. He hoped to prevent the Inquisition from condemning Copernican astronomy.
There is another change between the 1613 letter and the 1615 letter which indicates Galileo's awareness of a subtle theological distinction. In explaining that the purpose of the Bible is to lead men to salvation and not to disclose information extraneous to that purpose, Galileo writes the following to Castelli in 1613:
I would believe [Io crederei] that the authority of Holy Writ had only the aim of persuading [l'autorità delle Sacre Lettere avesse avuto solamente la mira a persuadere] men of those articles and propositions which, being necessary for salvation [sendo necessarie per la salute loro] and overriding all human reason [superando ogni umano discorso], could not be made credible by another science, or by other means than the mouth of the Holy Ghost itself.
In the letter of 1615, Galileo alters this passage; he writes:
I should judge that the authority of the Bible had the aim principally of persuading [l'autorità delle Sacre Lettere avesse avuto la mira a persuadere principalmente] men of those articles and propositions which, surpassing all human reasoning, could not be made credible by another science, or by any other means than through the mouth of the Holy Ghost.
In 1613, Galileo wrote that the purpose of the Bible was only [solamente] to persuade men of those truths which surpassed human reason. In 1615, he changes the adverb to "principally" [principalmente]; thereby, he does not exclude from the purpose of the Bible the revelation of truths which are within the realm of human reason. Notice, also, that the 1615 text omits the phrase "being necessary for salvation;" thus, Galileo eliminates a restriction concerning the subject of the articles and propositions which come under the "authority of the Bible." I think that these changes are significant for two reasons. First, with Aquinas, Galileo could now allow that some truths about God and man necessary for salvation which can be known by reason are also revealed in Scripture. Second, he admits that there may be truths in the Bible which are not directly connected to the Bible's purpose of leading human beings to salvation. Galileo is skilled in rhetoric. In order to persuade the Church not to act precipitously and condemn Copernican astronomy, Galileo, as we have seen, often makes it appear as though the new astronomy has already been demonstrated to be true. The Letter to the Grand Duchess is richly laced with quotations from the Church Fathers, principally Augustine, all left in Latin: passages which lend authority to his arguments. The passages quoted reinforce the general principles of the complementarity of science and scripture, and the need to avoid naive, literalistic interpretations of the sacred text.
In the absence of a scientific demonstration for the motion of the earth, Cardinal Bellarmino urged prudence: do not challenge the traditional readings of those biblical passages which have been interpreted as affirming the immobility of the earth. The Cardinal was acutely aware of Protestant challenges to the Catholic Church's claim to be the sole, legitimate interpreter of God's word. In fact, as I suggested in a previous lecture, it seems that Bellarmino was more concerned with maintaining the authority of the Church to be the authentic interpreter of Scripture than he was in refining principles of biblical exegesis. Nevertheless, on the level of the principles concerning the relationship between science and scripture, Cardinal Bellarmino and Galileo were in agreement, just as they were in agreement concerning the Aristotelian requirements for scientific knowledge.
Yet, there is something more in Galileo's arguments, more than the traditional affirmation that God is the author of the book of nature and the book of scripture and that the truths of nature and the truths of scripture cannot really be in conflict. In the passage in the 1615 Letter to the Grand Duchess which we have been examining, and elsewhere in the letter, we find an additional argument, an argument not found in his earlier letters on the subject. Let me quote three such passages in which Galileo argues for the role of science in discovering the true sense of those scriptural texts which address scientific questions.
...[Having become certain of any physical conclusions [venuti in certezza di alcune conclusioni naturali], we ought to utilize these as the most appropriate aids in the true exposition [alla vera esposizione] of the Bible and in the investigation of those meanings which are necessarily contained therein [quei sensi che in loro necessariamente si contengono], for these [meanings] must be concordant [concordi] with demonstrated truths [le verità dimostrate].
[Since] two truths cannot contradict one another [due verità non possono contrariarsi]. . . it is the function of wise expositors [of Scripture] to seek out the true senses [i veri sensi] of scriptural texts. These will unquestionably accord [indubitabilmente saranno concordanti] with the physical conclusions [conclusioni naturali] of which we are already certain and sure [certi e sicuri] through manifest sense or necessary demonstrations [senso manifesto o le dimostrazioni necessarie].
When one is in possession of knowledge about questions of nature which are not matters of faith, based on indubitable demonstrations or sensory experience, since such knowledge is also a gift from God, one must apply it to the investigation of the true meanings [veri sensi] of Scripture in those places which apparently seem to read differently. These senses would unquestionably be discovered by wise theologians [indubitatamente saranno penetrati da' sapienti teologi], together with the reasons for which the Holy Ghost sometimes wished to veil itself under words with a different meaning [velare sotto parole di significato diverso].
Galileo argues that there is not simply a complementarity between the Bible and science, in that the truth of one cannot contradict the truth of the other, but that there also must be a concordance between science and those passages in the Bible which appear to make claims about the physical nature of the universe.
There is a certain oscillation, or, perhaps, tension between Galileo's insistence that the Bible is extraneous to the natural sciences and his insistence that the Bible reflects the conclusions of the natural sciences. Galileo's principles were shared by his opponents in the Inquisition, although they reached a different conclusion when they examined the particular case of Copernican astronomy. The theological consultants of the Inquisition were asked to evaluate the claims of Copernican astronomy. They issued their report to the cardinals of the Inquisition in February 1616. As I discussed in a previous lecture, the report of the consultants concluded that the claim that the sun was immobile and at the center of the universe was "foolish and absurd in philosophy, and formally heretical since it explicitly contradicts in many places the sense of Holy Scripture, according to the literal meaning of the words and according to the common interpretation and understanding of the Holy Fathers and the doctors of theology." The theologians also concluded that the claim that the earth moves was foolish and absurd in philosophy and, "in regard to theological truth it is at least erroneous in faith." Many transcriptions of the report of the theological consultants fail to place a comma after the word "philosophia." The original Vatican manuscript (folio 42r) has a semicolon after "philosophia" and the late nineteenth century edition of the collected works of Galileo (19: 321) has a comma. Most translations into English omit the punctuation. Such a transcription, without a comma, "conveys the impression" that contradicting the Bible "is being given as a reason for ascribing both philosophical-scientific and theological heresy." But the comma between "philosophia" and "et" separates the claim of theological heresy from the claim of philosophical and scientific error. The distinction is crucial! For the original manuscript shows us that the theologians first conclude that the proposition is false and absurd philosophically and then conclude that it is heretical because it contradicts the Bible.
I want to point out, first, that these conclusions are conclusions of a committee of experts; they are not the official teaching of the Church. Even though the Inquisition, in dealing with Galileo, operated on the basis of this report, the report remains a committee report; it is not an official act of the Church. Too often historians of the Galileo affair fail to distinguish between such committee reports and formal decisions of the Church. And, as I suggested in my previous lectures, some scholars fail as well to distinguish between disciplinary and doctrinal decisions. It is important to note that the first part of each of the two conclusions reached by the theologians is that Copernican astronomy "is false and absurd" philosophically. Why should the theological experts of the Inquisition care whether Copernican astronomy is false scientifically? First of all, there is the ancient Catholic commitment to the safeguarding of reason since, as Aquinas would say, reason is a way to God. Aquinas, himself, will refer to those propositions about God, such as that He exists, which serve as preambles to faith. More importantly for our purposes, I think, is that these theologians were committed to the complementarity between science and scripture. If a proffered scientific proposition is false, scripture certainly cannot be in agreement with it, since the Bible cannot affirm as true that which reason knows to be false. Furthermore, in reaching the conclusion that Copernican astronomy contradicts the Bible, the theologians accepted as incontrovertibly true a particular geocentric cosmology, and, on the basis of such an acceptance, they insisted that the Bible be read in a certain way. Thus, in part, they subordinated scriptural interpretation to a physical theory. They proceeded in this manner because, like Galileo, they were convinced that the Bible contained scientific truths and that, on the basis of what is known to be true in the natural sciences, one could discover the same truth in related biblical passages.
Allow me to conclude this brief account with a broader historical claim. As William Wallace, A.C. Crombie, and others have shown, Galileo's scientific achievements must be recognized in the setting of a progressive Aristotelian science; that is, Galileo's arguments in physics, including his refutation of Aristotelian conclusions about the immobility of the earth, proceed from first principles of Aristotelian natural philosophy. Similarly, Galileo's theological claims are part of the traditional heritage of Catholicism, and, further, they are a part of the theological environment of the Counter Reformation Church. The Council of Trent's injunctions concerning the proper reading of Scripture are recognized by both Galileo and the Inquisition. Let us recall, further, that a crucial feature of the disputes of the Reformation was the calling into question by the Reformers of the very criterion of truth by which one resolves theological questions. In other words, the Reformation was not simply a debate about grace, free will, predestination, and the like, but it also involved a debate about the Catholic Church's claim to be the authentic judge of such disputes.
Although Protestants and Catholics would disagree about the role of the Church as a criterion of truth, they could, however, and they did, appeal to a common text, the Bible: a text, which, in a sense, standing alone, served as the only common ground from which to argue. Both sides, thus, were encouraged to find in the Bible evidence for their respective theological conclusions. The Bible, therefore, came to be treated as a reservoir of conflicting theological propositions. Thus, we find a tendency on the part of both Protestants and Catholics to treat the Bible as a theological text book: a compendium of syllogisms or dogmatic propositions. One of the obvious dangers in viewing the Bible as a text book in theology is a literalistic reading of the text: a literalism all too apparent in the Inquisition's reaction to the perceived threat of the new astronomy. Do we not see a similar tendency in Galileo's insistence that we can discover scientific propositions in the Bible? Armed with scientific demonstrations we, or at least wise expositors, possess the key to discover those scientific propositions which are contained in the Bible. Despite the commonly accepted view of most scholars, Galileo's principles of biblical interpretation do not anticipate a modern distinction between science and an essentially religious reading of the Bible. Galileo the theologian reaffirms not only the ancient traditions of Catholic theology, but manifests as well theological tendencies in some sense peculiar to his age.
For a fuller treatment of the theme of this lecture, see: William E. Carroll, "Galileo and the Interpretation of the Bible," Science & Education 8:2 (1999), pp. 151-187.
