Aristotle ends the Physics with a formidable book devoted to proving the existence of a first principle of motion. This, it may be noted, is not the only place in his writings where he applies himself to the task. Three times, in fact, we find him bending his efforts to it, twice in the Physics (VII, i, and VIII, virtually in toto), and once in the Metaphysics (A, 6). The contents of Physics VII, however, are really an abbreviated version of Book VIII, and all indications point to Book VII not having been in the original redaction. But the other two occurrences, Physics VIII and Metaphysics A, 6, do not simply repeat each other. They are distinct presentations of the demonstration at hand, and a comparison of them raises two questions of major importance.
1) One is whether the prime mover of Book VIII is to be identified with the first substance, with pure act, the burden and conclusion of the argument in the Metaphysics? Granted that the demonstrations themselves are basically alike, the end-products, the things demonstrated, look different. In the Physics we come at last upon a physical prime mover, unextended and immaterial no doubt, but having no other function, it seems, than to move the first sphere of the heavens. Is this God? Or is it only a transcendent physical mover? In the Metaphysics, on the other hand, the supreme principle arrived at has all the attributes of a first and unique being; it is pure act, thought of thought, etc.
The question, as we have said, is whether these two principles, the prime mover of the Physics and the pure act of the Metaphysics, are identical. The answer is undoubtedly yes, but with a proviso. In the Physics the prime mover, formally speaking (that is, on the evidence of the demonstration), is merely shown to be the first physical principle of motion in the universe; whereas in the Metaphysics the prime mover is brought forth with all the properties of the primary, absolute being.
2) The second question, or problem, is more difficult to decide. The prime mover of the Physics acts in the manner of an efficient cause, but the prime mover of the Metaphysics is said to put the spheres in motion by being the primary object of desire, hence as final cause. These two points of view are not necessarily opposed; in the Christian universe, as a matter of fact, the one complements the other. But Aristotle's philosophy wants a complete explanation of the relationship between God and the world, and this lack poses the problem how to reconcile the prime mover as efficient cause with the prime mover as final cause. The solution, however, is beyond the scope of this volume. We turn at once, therefore, to the demonstration in the Physics. Since the succession of arguments that make up this demonstration is rather extensive, and the reasoning close and involved, we shall not make a point-by-point examination of it. Indeed, such a course would likely defeat the purposes of an introductory study. Suffice it, then, to produce the basic structure of the demonstration and, in conclusion, to indicate the recasting it undergoes in the hands of St. Thomas.