1. Definition of Motion
a) In Book III Aristotle makes no reference to the Eleatic theory (i.e., the denial) of motion. Having once for all affirmed its existence in Book I, his only concern now is to explain its nature. Also, he makes short shrift of the opinion that motion is a separate reality, after the Platonic fashion. Motion, he insists, is something of the physical world; it is in things themselves, and has to be explained in the light of its sensible manifestations.
Nevertheless, for his definition of motion, which he advances with a minimum of preliminaries, Aristotle calls upon a cardinal distinction of the metaphysical (rather than the physical) order, the distinction of act and potency. He could hardly do otherwise, seeing that motion is a fundamental concept, above classification in any single predicament, for it is found in several. Hence, if it is to be defined at all, recourse must be had to transcendental notions. And this is what Aristotle does.
b) Granted the distinction of act and potency, what is in potency is not yet in motion: a thing that is not yet being warmed, is not in motion toward warmth. On the other hand, what has reached its term, what is in completed act, is not in motion either: a thing that is warm is no longer in motion toward warmth. Consequently, to be in motion is to be in an intermediate state, between the initial potency and the terminal act, hence partly in potency and partly in act. In a word, the warmth of a thing that is being warmed is in imperfect act, and this imperfect act is motion, but on condition that the thing remained headed toward further warming. Motion impounds, as it were, both ideas, both act and potency at once. In Aristotle's celebrated definition it is "the entelechy (act) of what exists in potency, so far as it is in potency." 2 Or, in the Latin of the Schoolmen, it is:
actus existentis in potentia in quantum est in potentia.
This definition, which has often been abused but never improved, could be discussed at great length. For the present, note three points.
Actus (act) indicates that motion itself is a kind of fulfillment or realization; in the warming of a thing there is already a degree of actualization present.
Existentis in potentia (of what exists in potency) affirms that the act in question is not at a standstill, as though fully realized, but that the subject of the act remains in potency to still more actualization.
In quantum est in potentia (so far as it is in potency) means that the act identified with motion determines or actuates its subject, not in every respect, but in the respect by which it is in potency. So, for example, in the carving of a statue the process of carving is not the actualization of bronze as bronze, but of bronze so far as it is in potency to becoming a statue.
These points we find recapitulated, with characteristic skill, in St. Thomas, who writes:
Thus, imperfect act has the nature [ratio] of motion either way, that is, whether we take it as potency in respect to further act, or as act in respect to something yet more imperfect. Hence, it is neither the potency of what exists in potency nor the act of what exists in act, but the act of what exists in potency. So, saying it is "act" we designate its order to the anterior potency, and saying "of what exists in potency" we designate its order to the ulterior act.3
c) Motion, then, however we approach it, is imperfect act, a potentiality not yet perfectly actualized. It is, as remarked, a kind of intermediate state between complete potency and complete act. Aristotle takes special note of motion's intermediateness and incompleteness. "Motion," he says, "is indeed a kind of act, but incomplete, and the reason is that the thing in potency whose act it is, is incomplete.4
To be sure, Aristotle was not the first to mark the indeterminate character of motion, or to wrestle with it. Others had ruminated on it but never achieved a philosophically technical explanation. St. Thomas, on the other hand, though not the author of the definition of motion, shows in his analysis a penetration to rival his master's. If motion is actus imperfectus, it differs from things that are fully realized, fully in act, and St. Thomas has talented paragraphs on the manner of this difference.5 If to the definition there nevertheless clings a veil of obscurity and of impenetrability, this is not because the definition is faulty but simply because motion itself is, as Aristotle intimates, a shadowy thing, hovering as it were between light and darkness, between act and potency!6