known in itself when the attribute belongs to the essence of the subject, as when I say, man is an animal; for the idea of animal is necessarily contained in that of man. But if the attribute and the subject be known to all men, then will the proposition be also understood by them, as is evidently true of the first principles of demonstration. . . . I say, therefore, that this proposition, 'God exists,' is known in itself intrinsically, because the attribute is one with the subject, since God is his own existence." Yet the Angelic Doctor is careful to mark the limits of human intelligence, while he avoids the unreasonable error of the modern agnostic. He proceeds: "But because we do not know what God is, the proposition is not in itself known to us, but needs demonstration from what is better known to us, albeit less known in itself. In a word, God's existence is demonstrated from the effects which he has produced" (Sum. th., I, q. 2, a. 1). Were St. Thomas to stop here, the atheist of whatsoever guise might justify his own unbelief. But our Doctor, though a most docile child of Mother Church, has learned that he owes to God a rational service. He has been taught that both faith and reason proceed from the Most High, and he who denies to reason its native power detracts from the goodness of its infinite Author. "Can man demonstrate the existence of God?" Can the finite compass the Infinite? Let us note the caution with which the Master approaches the solution of this question of tremendous moment. "Demonstration," he says, "is of two kinds: one from cause, and called propter quid, or a priori (in the scholastic sense of the term); the other is from effect and is known as demonstration quia, or a posteriori. . . . Since the existence of God is not known to us from the mere consideration of the terms, it can be demonstrated only from its known effects." And forthwith he proceeds to establish by five powerful arguments the existence of a Supreme and Infinite Personal Being.—The first proof is derived from motion. "It is certain," argues St. Thomas, "and palpable to sense that there are in this world things that are subject to motion. Now, whatever receives motion is moved by something
