external; for nothing is moved except in so far as it is in potentiality to that term to which it is moved; and that which causes motion in another acts in so far only as it is itself in act. For to cause motion in anything is nothing but to reduce it from potentiality to act. But a thing can be reduced from potentiality to act by that only which is in act. . . . Now, it is impossible for the same thing to be in potentiality and in act in the same respect. . . . Therefore it is impossible for a thing to be in the same respect. . . . causing motion and receiving motion, or for it to move itself. Therefore whatever is moved is moved by another. But if that which causes its motion be also moved, it likewise must be subjected to something else, and this in turn to another. We cannot, however, proceed thus ad infinitum; for were this possible, there would be no first mover, and consequently no other causes of motion, for second causes of motion act only from an impulse received from the prime mover. . . . We must then arrive at a first mover unmoved; and this is what all men call God (Sum. th., I, q. 2, a. 3).—This proof, the grandest adduced in support of God's existence, is drawn from Aristotle (Met., XI, 8) who defines motion as the "act of an entity existing in potentiality as such." Motion in this sense may therefore be expressed as "any actuation of a passive power." So vast is its extension that it includes not only all the changes of the physical world, but also all the actions of man as man, his intellections and volitions. But just as Aristotle's definition is illustrated best by locomotion, so the Angelic Doctor lays special stress upon the changes palpable to sense in view of man's dependence, extrinsic though it be, upon the things of sense for the performance of even his most spiritual actions; and bases his reasoning upon the incontrovertible principle of contradiction. The changes that are effected round about us would be inconceivable were not their respective subjects in potentiality to be changed. "Ab esse ad posse valet illatio." But that which causes a change must have actuality or be in act, since act is a perfection and the first perfection of an entity is existence; or, as St. Thomas expresses it (Sum. th.,
