the principles of sound philosophy, and that the proof of the conclusion to which he objects belongs to the domain of ontology.
St. Thomas' third proof of the existence of God starts from the relation of the contingent or possible to the necessary. "We find in nature," he says, "that some things are capable of existence and of non-existence, for we see that they are subject to generation and corruption. But such things cannot possibly be always in existence, for what is only possible to be has a time of non-existence. Therefore, if all things are contingent, there was a time when nothing existed. Were this true, nothing would even now exist, because what is not can be actuated by that only which is. . . . Therefore not all things are contingent, but something is necessary being. Now, every such being either has or has not an extrinsic cause of its existence. But we cannot grant an infinite series of dependent necessary beings. . . . Therefore, we must grant something which by itself is necessary, not as dependent on an extrinsic cause, but as causing the necessity (hypothetical) of other beings. This cause is God." So far St. Thomas.
We have already briefly referred to the principles of Actuality and Potentiality, which underlie this demonstration; but we must now consider them in their relation to Existence and Essence, to Form and Matter. "The Possible," says our illustrious Schoolman, "has a twofold signification, as the philosopher teaches (Metaphys., V). One includes relation to some power; as that which is subject to human power, is said to be possible for a man. The other is absolute, having regard to the simple congruity of the terms" (Sum. th. I, q. 25, a. 3). And again (Contra gentiles, I, 16): "Everything of whose substance potentiality is a component, according to its degree of potentiality, is capable of non-existence; because what is merely capable of being is also capable of not being." The same thought he expresses in Lib. II, c. 53: "In whatsoever entity are found two principles, one of which is the complement of the other, the proportion of one to the other is as that of potentiality to act, for nothing is completed
