Commentary Aristotle Physics
Lecture 2. The definition of motion
We have now before us the distinctions in the various classes of being between what is fully real and what is potential. The entelechy of a thing existing in potency, insofar as it is such, is motion— namely, of what is alterable qua alterable, alteration; of what can be increased and its opposite, what can be decreased (there is no common name), increase and decrease; of what can come to be and can pass away, coming to be and passing away; of what can be carried along, locomotion.
Examples will elucidate this definition of motion. When the buildable insofar as it is buildable is in act, it is being built, and this is building. Similarly with learning, doctoring, rolling, leaping, ripening, aging, and so on.
The same thing, if it is of a certain kind, can be both potential and in act, not indeed at the same time or in the same respect, but for instance, potentially hot and actually cold.
Hence, such things will at once act and be acted on by one another in many ways: each of them will be capable at the same time of causing alteration and of being altered.
Hence, too, what effects motion as a physical agent can be moved: when a thing of this kind causes motion, it is itself also moved.
This, indeed, has led some people to suppose that every mover is moved. But this question depends on another set of arguments, and the truth will be made clear later.
It is possible for a thing to cause motion while it is itself incapable of being moved. Motion is the act of a thing in potency when it is already in act and operates not as itself but as movable.
What I mean by “as” is this: bronze is potentially a statue. But it is not the act of bronze as bronze that is motion. For “to be bronze” and “to be a certain potentiality” are not the same. If they were identical without qualification, meaning in definition, the act of bronze as bronze would have been motion. But they are not the same, as has been said.
(This is obvious in contraries. “To be capable of health” and “to be capable of illness” are not the same, for if they were, there would be no difference between being ill and being well. Yet the subject both of health and of sickness—whether it is humor or blood—is one and the same.)
We can distinguish, then, between the two—just as, to give another example, “color” and “visible” are different—and clearly it is the act of the possible insofar as it is possible that is motion. So this, precisely, is motion.
283. After first setting down certain things necessary for investigating the definition of motion, the Philosopher now defines motion:
first, in general;
second, more specifically, at “what motion is, then” (202b23; [325]).
With regard to the first, he does two things:
first, he shows what motion is;
second, he inquires whether motion belongs to the mover or to the mobile thing, at “the mover is also moved” (202a3; [297]).
As to the first of these, he does three things:
first, he gives the definition of motion;
second, he explains the parts of the definition, at “examples will elucidate” (201a15; [287]);
third, he shows that it is a good definition, at “further, it is evident” (201b5; [292]).
As to the first, he does two things:
first, he gives the definition of motion;
second, he gives examples, at “namely, of what” (201a11; [286]).
284. As to the first, one must understand that some have defined motion by saying that motion is a going-out from potency to act that is not sudden. But they are found to be in error, because they have placed in the definition certain elements that are posterior to motion: for a going-out is a species of motion; sudden likewise involves time in its definition (the sudden is that which occurs in the indivisible unit of time); time, however, is defined in terms of motion.
285. Consequently, it is entirely impossible to define motion in terms of what is prior and better known otherwise than the Philosopher here does. For it has been pointed out already that every genus is divided by potency and act. Now, potency and act, since they are among the first differences of being, are naturally prior to motion, and it is these that the Philosopher uses to define motion.
Consider, therefore, that something is in act only, something is in potency only, and something else is midway between potency and act. What is in potency only is not yet being moved; what is already in perfect act is not being moved, but has already been moved. Consequently, that is being moved that is midway between pure potency and act, which is partly in potency and partly in act—as is evident in alteration. For when water is only potentially hot, it is not being moved; when it has now been heated, the motion of heating is finished; but when it possesses some heat, though imperfectly, then it is being moved, since whatever is being heated gradually acquires more and more heat.
Therefore, this imperfect act of heat existing in a heatable object is motion—not, indeed, by reason of what the heatable object has already become, but inasmuch as, being already in act, it has an order to a further act. For should this order to a further act be taken away, the act already present, however imperfect, would be the term of motion and not motion itself—as happens when something becomes half-heated. This order to a further act belongs to the thing that is in potency to it. Similarly, if the imperfect act were considered solely as ordered to a further act, under the account of potency, it would not have the account of motion, but of a principle of motion—for heating can begin from either a cold or a lukewarm object.
The imperfect act, therefore, has the character of motion both insofar as it is compared, as potency, to a further act, and insofar as it is compared, as act, to something more imperfect. Hence, motion is neither the potency of a thing existing in potency nor the act of a thing in act, but it is the act of a thing in potency, where the word act designates its relation to a prior potency and the words of a thing in potency designates its relation to a further act. Hence the Philosopher most aptly defines motion as the entelechy, the act, of a thing existing in potency, insofar as it is such.
286. Then, at “namely, of what” (201a11), he gives examples from all the species of motion, such as that alteration is the act of the alterable insofar as it is alterable.
And, because motion in quantity and in substance does not have a single name in the same way as motion in quality is called alteration, he gives two different names for the motions in quantity and says that the act of the increasable and of its opposite, the decreasable (for which two there is no common name), is increase and decrease.
Similarly, the acts of the generable and of the corruptible are generation and corruption, and the act of what is mutable in regard to place is called change of place.
In this section, the Philosopher uses the word “motion” for any kind of change and avoids the strict usage in which motion is distinct from generation and corruption, as will be said in book 5.
287. Then, at “examples will elucidate” (201a15), he explains the several words of the definition:
first, he explains the use of the word act;
second, at “the same thing” (201a19; [288]), he explains of a thing existing in potency;
third, at “what I mean” (201a29; [289]), he explains insofar as it is such.
As to the first, he uses this reasoning.
That by which something previously existing in potency becomes actual is an act. But something becomes actual when it is being moved, although previously it was in potency. Therefore, motion is an act.
He says, therefore, that it is plain that motion is this, namely, an act, from the fact that the buildable implies a potency to something but, when the buildable is being reduced to act according to this potency that it implies, we then say it is being built—and this act is passive building. And the same thing is true in all other motions such as being taught, healing, rolling, dancing, growing up (that is, increase), growing old (that is, decrease), and so on.
For it must be remembered that, before something is being moved, it is in potency to two acts: to a perfect act that is the term of the motion, and to an imperfect act that is motion itself. Thus, before it begins to be heated, water is in potency to being heated and to having been heated. When it is being heated, it is being reduced to the imperfect act that is motion, but not yet to the perfect act that is the term of the motion—rather, in respect to this, it still remains in potency.
288. Then, at “the same thing” (201a19), he shows that motion is the act of a thing existing in potency. For every act is strictly the act of that in which it is always found—as light is never found but in the transparent, for which reason it is the act of the transparent. But motion is found always in a thing existing in potency. Therefore, motion is the act of a thing existing in potency.
To explain the second proposition, he says that, since certain things are the same both in potency and in act—although not at the same time or in the same respect (as, for example, something is hot actually and cold potentially)—it follows that many things mutually act and are acted upon, insofar as both are in potency and in act with respect to the other under different aspects. And, because all lower natural bodies share the same matter, there is therefore in each of them a potency to what is actual in another. Hence, in all such bodies, something simultaneously acts and is acted upon, both moves and is moved.
This fact had led some to say absolutely that every mover is likewise being moved. This point will be cleared up in a later place. For it will be shown in Physics 8 and in Metaphysics 12 that there exists an immobile mover, since it is not in potency but in act only. But, when that which is in potency, yet existing in a certain way in act, either acts itself or is acted upon by another insofar as it is movable, that is, is reduced to the act of motion—whether moved by itself or by another—then motion is its act. That is why things in potency, whether they act or are acted upon, are moved, since they are acted upon when acting and are being moved when moving—just as fire, when it acts on logs, is acted upon, insofar as it becomes more dense through smoke (since flame is nothing more than burning smoke).
289. Then, at “what I mean” (201a29), he explains this part of the definition, insofar as it is such:
first, by an example;
second, by giving a reason, at “this is obvious in contraries” (201a34; [290]).
He says first (201a29) that the phrase insofar as it is such had to be added because what is in potency is at the same time something in act. And, although the subject that is both in potency and in act may be the same, nevertheless to be in potency and to be in act is not contained under the same account.
Thus, although brass is a statue in potency but is brass actually, nevertheless the account of the brass as brass is not the same as the account of the brass as it is in potency to a statue.
Now, motion is not an act of the brass insofar as it is brass, but insofar as it is in potency to a statue; otherwise, during the whole time that it was brass, it would be undergoing motion, which is clearly false. That is why it is necessary to add insofar as it is such.
290. Then, at “this is obvious in contraries” (201a34), he explains the same thing by using an argument based on the nature of contraries. For it is clear that a given subject is in potency to contraries—as a humor or the blood is in potency to health and to sickness. But to be in potency to health is one thing and to be in potency to sickness is another, if one considers their objects. Otherwise, if to be able to be sick and to be able to be well were the same thing, it then would follow that being sick and being well would be the same. Hence, the idea of “being able to be sick” differs from “being able to be healthy,” although their actual subject is one and the same thing.
It is plain, therefore, that there is not one and the same account of the subject as it is a certain being and as it is in potency to something else. Otherwise, potency to contrary things would fall under one and the same account. In like manner, the account of that which is color and that which is visible are not one and the same. Thus, it was necessary to say that motion is the act of the possible insofar as it is possible—to prevent supposing that it is the act of what is in potency insofar as it is merely some subject.