The third way of demonstrating God’s existence is from contingency. It is closely related to the demonstration from efficient causation – the second way.
Aquinas states the third way:
“The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence — which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.”
Before going further, it might be helpful to define some terms. A contingent being is one that need not exist. In other words, it may exist or it may not exist. Experience shows me that such contingent beings do indeed exist. I clearly see things come into existence and then pass out of it. Animals, plants, humans all have their life spans – being born, growing and developing, and then dying. Inanimate substances enter into compositions forming new substances with properties different from those of its constituents. After a time, the compound is again resolved into its original elements.
Necessary being is that which is incapable of not existing. Actual existence belongs to the very nature of necessary being just as to be three-sided belongs to the nature of a triangle. Actual existence is an essential attribute of necessary being. A necessary being must be self-existent.
Since contingent being may either exist or not exist, it is obvious that it cannot account for its own existence. If it was the sufficient reason for its own existence then it would be impossible for it to ever not exist – it would not be contingent. There must be some sufficient reason for its existence, and since it cannot be its own sufficient reason, we must look to something outside of it to explain its existence.
The conclusion is the same, even if we suppose a contingent being to have existed from all eternity. Once we admit that a thing is capable of non-existence, it follows of necessity that it owes its existence to something else, and this in the last resort, must be necessary being.
So the real question is what is “necessary being”. Modern materialism may answer that although individual substances are contingent, the whole vast collection of such substances taken in its entirety could be regarded as necessary. In other words, it is true that beings we experience all around us are contingent – animals, plants, rocks, humans, etc. But the universe as a whole can be regarded as necessary. That while the individual components and things contained within the vast universe are contingent, the universe is not. After all it is a scientific law that matter cannot be created or destroyed. So is this matter, the substratum common to all the existing things we observe in the universe, the necessary being we are searching for?
The answer, quite simply, is “no” for two reasons.
First, no collection or series of contingent beings, even if the series was infinite as regards time or space, could ever constitute a necessary being. If each member of the whole is contingent, then the whole, no matter its size or scope, does not contain within itself any sufficient reason for its existence – it simply cannot be self-existent.
Contingency is an attribute belonging to the essential nature of the object of which it is affirmed. Such attributes are predicable not merely of individuals but of the whole collection.
Likewise, the supposition that matter is the necessary being can be shown to be impossible. Matter is capable of receiving perfections that are not among its essential attributes. Matter can become a man, or gold, or a flower, etc. Each of these substances (man, gold, flower) has distinctive properties. But each of these properties is not essential to matter – for what is essential cannot be separated from it. So where do these properties come from?
Since they are not essential to matter, they must arise from the operation of some cause external to the matter. This leads us to a principle of prime importance, i.e., whenever two things essentially distinct one from the other are found in union, this must be due to the operation of an efficient cause outside and other than the things themselves. If two things essentially distinct, A and B, are found united or combined the reason cannot be found in A. A is the sufficient reason only for itself and its own essential attributes. Likewise it cannot be found in B. For example, think of a triangle. No cause is needed to explain why a triangle should have three angles. The attribute of having three angles results immediately from the essence of a triangle as a plane figure bounded by three straight lines. You simply cannot have a triangle without three angles. Now think of a wooden triangle. Wood is not triangular necessarily. Nor is a triangle necessarily wooden. The composition of these diverse elements of necessity supposes the operation of an efficient cause other than and outside the wood and triangle.
Necessary being is self-sufficient and self-existent. No agent can exist capable of conferring perfections upon necessary being. For this to be so, the agent must be other than and outside the necessary being. But such being other than and outside necessary being can only be contingent being. But contingent being owes whatever attributes it possesses in last resort to necessary being. Contingent being has nothing that is not already found in necessary being and cannot add anything to it. It follows then that matter which is a recipient for perfections cannot be necessary being.
“But”, it may be argued by some, “what if there are a plurality of necessary beings?” “Wouldn’t that invalidate the argument you just stated?”
A plurality of necessary beings is nonsensical . Let’s suppose there are two necessary beings, A and B. To be separate means they must in some way be distinct one from another. To be distinct means that A must have attributes not found in the B. But such attributes that provide this distinction are not essential to A – for if they were essential to the necessary being of A than they must, by definition, be found in the other necessary being B - which of course would dissolve any distinction. Furthermore if the attributes are not essential to A, they must arise from a cause other than and outside A. But if A is necessary being than it is self-existent and self-sufficient– meaning that no agent can exist capable of conferring perfections upon it. Thus the very nature of necessary being means that it can only be one.
Equally invalid is the pantheist contention (common to many Eastern religions as well as the New Age movement in the West) that contingent beings are merely modes of the one and all-inclusive Absolute - that they are manifestations of necessary being, and not entities possessed of a distinct though dependent existence. Here we may make appeal to the argument which we have just employed in regard of material substances. The contingent beings of experience are constantly undergoing changes and acquiring new perfections. This alone establishes that they are not modes of necessary being. A sheer contradiction is involved in the supposition that an agent exists, which can confer perfections on necessary being. Yet the acquisition of a perfection apart from an agent is, as we have seen, a metaphysical impossibility.
Thus, Aquinas states simply:
“Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity.
This all men speak of as God.”
