4. The Prime Mover Is Without Magnitude 15
That the prime mover is devoid of all magnitude is reasoned as follows. Its magnitude would be finite or infinite. But we know from the preceding chapter that a magnitude cannot be actually infinite. As for a finite magnitude or mover, it could not impart an infinite motion: this would be a contradiction. Consequently, if the motion imparted by the prime mover is eternal, which means infinite, the prime mover must be without magnitude, hence indivisible and without parts. Such is Aristotle's conclusion, the importance of which is seen at once. The prime mover, it follows, is not in the order of quantified beings, therefore is not, it would seem, a material reality. But to call it immaterial is merely to put it negatively. What it is positively speaking the Physics does not tell. For the answer we must go to the theological portions of the Metaphysics.16 There we are informed that only pure act, posited as the ultimate principle of the universe, meets all the requirements of an absolutely prime and primary being.17
5. Conclusion: Reflections on Aristotle's Demonstration and Comparison with the "Prima Via" of St. Thomas
a) First, a few comments on the method or procedure employed by Aristotle. This, as one cannot but notice, bears a strongly a priori stamp. There is, to be sure, constant reference to the given, to the vouchings of sense, so that the image of the universe that finally unfolds does conform with experience. But if Aristotle is anxious to square his design of cosmic movers and movables with experience, the impression persists that he is even more solicitous about something else. What he wants to prove above all, or so it seems, is that this is the system of motion which the universe, to be perfect, must have.
If this impression be correct, to what extent can his argumentation be considered valid? Some parts, no doubt, hang by an improbable thread, as when, to say no more, he argues the case for circular motion. Other parts would just as certainly have to be trimmed out. But there is no simple rule for pruning his dialectic. His demonstration of the prime mover is not, to change the metaphor, of a piece but textured, comprising, as we have seen, many separate arguments. Each of these arguments must be judged on its own merits, but only after the detailed analysis that does not come within our purview.
Yet, however the verdict should turn out on this or that particular aspect of the demonstration, underlying it all are two philosophical principles which seem unimpeachable. These principles are "that whatever is moved, is moved by another," and "it is impossible for a series of moved movers to be infinite." If these be true - and no one has successfully challenged them - Aristotle's demonstration is fundamentally sound and invulnerable, as St. Thomas, for one, was quick to perceive.
b) This brings us to the second point, Aristotle and the prima via. St. Thomas takes up Aristotle's proof of the prime mover on several occasions, sometimes merely expounding it, as in the commentaries," other times adapting it to his purpose, as in the two Summas.19 With St. Thomas the demonstration had, of course, to undergo an important modification, since the fact of creation in time ruled out Aristotle's initial supposition of motion being eternal. This, however, does not undermine the demonstration; in fact, says St. Thomas, the causality of the prime mover is made the more evident by the world having a beginning.20 Just the same, Aristotle's proof comes out considerably revamped.
Especially worthy of note, for example, is the way St. Thomas, in the Summa theologise, untangles the argument of the Physics from the whole machinery of Aristotle's physical universe. As for the two basic principles mentioned earlier, on which Aristotle's proof rests, these could not but be retained. St. Thomas, however, does not validate them in the manner of Aristotle in the Physics; instead, his whole defense of them is embodied in a pair of more fundamental propositions, namely, "that a being cannot be reduced from potency to act except by something that is in act" and "where there is no first term, there can be no ultimate or intermediate term." 21 Thus, while it is true that in the hands of St. Thomas Aristotle's proof remains metaphysically the same, nevertheless there is performed by St. Thomas a process of enucleation that brings the core of the argument more plainly to light.
To illustrate, may we cite the text of the prima via, a text in which, it may be added, the long history of natural philosophy achieves, as it were, its crown and pinnacle. For this reason it not only makes a fitting conclusion to the present discussion but also qualifies to be cited in full:
That God exists can be proved in five ways. The first and more evident way is the one taken from motion. It is certain and evident to the senses that some things in this world are in motion. Now, whatever is in motion, is moved by another; for nothing moves except so far as it is in potency to the thing toward which it is moving. On the other hand, whatever imparts motion does it so far as in act; for to move something is none other than to bring it from potency to act, and nothing can be reduced from potency to act except by something in act. Thus, what is actually hot, say fire, causes wood, which was potentially hot, to be actually hot, and thereby moves and alters it. It is not possible, however, that the same thing should be simultaneously in act and potency in the same respect, but only in different respects. For example, what is actually hot cannot at the same time be potentially hot, but it is at the same time potentially cold. Impossible, therefore, that in the same respect and in the same manner a thing should be both mover and moved, in other words, move itself. Hence, whatever moves [i.e., is in motion] is necessarily moved [i.e., put in motion] by another. And if the thing by which it is moved, is itself in motion, then this also must be moved by another, and this again by still another. But this regression cannot continue to infinity, because then there should be no first mover, and consequently no other mover, the reason being that second movers move only through being moved by the first mover, as for example a walking-stick moves only through being moved by the hand. Consequently, it is necessary to come to a first mover that is moved by no other, and this all understand to be God.22