Commentary Aristotle Physics
Lecture 4
Motion is the act both of the mover and of the mobile object
The mover is also moved, as has been said—that is, every mover that is in potency to being a mover, and whose immobility is rest—when a thing is subject to motion, its immobility is rest.
For to act on the movable as such is just to move it. But this it does by contact, such that it is also acted on at the same time.
Hence, we can define motion as the act of the mobile inasmuch as it is mobile, the cause of this attribute being contact with what can move such that the mover is also acted on.
The mover or agent will always be a form, either a “this” or “such” or a “quantity,” which, when it acts, will be the source and cause of the change; for instance, the fully formed man begets man from what is potentially man.
The solution of the difficulty that is raised about the motion—whether it is in the movable—is plain. It is the act of this potentiality by the action of that which has the power of causing motion.
And the act of that which has the power of causing motion is not other than the act of the movable.
For it must be the act of both. A thing is capable of causing motion because it can do this; it is a mover because it is in act.
But it actualizes the mobile. Hence, there is a single act of both alike, just as one to two and two to one are the same interval, and the steep ascent and the steep descent are one—for these are one and the same, although they can be described in different ways. So it is with the mover and the moved.
297. After defining motion, the Philosopher now shows whose act motion is, whether it is the act of the mobile or of the mover.
Also, it may be said that he gives another definition of motion that is related to the previous one as material to formal and as a conclusion to its principle.
And this is the definition: motion is the act of the mobile inasmuch as it is mobile. This definition is a conclusion from the previous one. For since motion is the act of a thing existing in potency inasmuch as it is in potency, and since that which exists in potency as such is the mobile and not the mover (for the mover as such is in act), it follows that motion is an act of the mobile as such.
298. In regard to the main question, he does three things:
first, he shows that motion is an act of the mobile;
second, he shows how motion is related to the mover, at “and the act of that” (202a15; [304]);
third, he raises a difficulty, at “this view has a rational defect” (202a21; [308]).
About the first, he does two things:
first, he posits a definition of motion, namely, that motion is an act of the mobile;
second, he clears up a doubt, at “the solution of the difficulty” (202a13; [303]).
In regard to the first, he does three things:
first, he investigates the definition of motion;
second, he gives the definition, at “hence, we can define” (202a7; [302]);
third, he explains it, at “the mover or agent” (202a7; [302]).
In investigating the definition, he shows that to be moved happens even to the mover.
In regard to this, he does two things:
first, he shows that every mover is moved;
second, he shows why that happens, at “for to act” (202a5; [301]).
299. He shows in two ways that the mover is moved.
This is so first of all because anything that is previously in potency and then in act is somehow moved. But movers are found that are previously movers in potency and afterward movers in act; therefore, this kind of mover is moved. He states, therefore, that every mover, since it is such that it is in potency to being a mover, is likewise moved. This is clear from what has been said, for it was said that motion is an act of a thing existing in potency, and this occurs in every natural mover; that is why it was said above that every physical mover is moved.
300. Second, at “and whose immobility” (202a4), he brings out the same point in another way: whatsoever’s immobility is its rest is capable of motion, for rest and motion, since they are opposites, happen to the same things. But the immobility of a mover—that is, its ceasing from moving—is called rest, for there are things that are said to rest when they cease to act. Therefore, every such mover—one whose immobility is rest—is moved.
301. Then, at “for to act” (202a5), he shows why it happens that a mover is moved. For it does not happen precisely because it is a mover, but because it is such by touching. For to move is to act so that something may be moved, and what is so acted upon by the mover is moved. But whatever acts does so by touching, for bodies act by touching; hence it follows that what acts is at the same time acted upon, because that which touches is acted upon.
However, this must be understood of those cases where there is mutual touching—that is, when the thing touching is also touched, as happens in things that are material, where both of the things are acted upon when they touch one another.
But heavenly bodies, because they do not have the same kind of matter as the lower bodies, so act on them that they are not acted upon in return, and they touch without being touched, as is stated in De generatione 1.
302. Then, at “hence, we can define” (202a7), he posits the definition of motion, concluding from what has been said that, although the mover is moved, motion nevertheless is not an act of the mover, but of the mobile inasmuch as it is mobile. He shows this subsequently by the fact that to be moved is accidental to the mover and does not belong essentially to it. Hence, if something is moved precisely inasmuch as its act is motion, it follows that motion is an act not of the mover but of the mobile—not, indeed, insofar as it is a mover but insofar as it is a mobile.
That to be moved is accidental to the mover is clear from what was pointed out in the earlier part of this lecture; for this act of the mobile, which is motion, happens from its contact with the mover. From this, it follows that, at the same time that it is acting, it is acted upon, and thus to be moved is accidental to the mover.
That to be moved does not belong essentially to the mover is clear from the fact that some form is always seen to be the mover: the form that is in the genus of substance is the mover in substantial change; a form in the genus of quality is the mover in alteration; and a form in the genus of quantity is the mover in growth and decrease. Forms of this type are the causes and principles of motions, since every agent moves according to its form. For every agent acts inasmuch as it is actual, as an actual man makes an actual man of man in potency. Hence, since it is through its form that a thing is actual, it follows that form is the moving principle. Thus, to move belongs to a thing inasmuch as it has a form through which it is actual. Therefore, since motion is the act of a thing existing in potency, as was said above, it follows that motion belongs to a thing not insofar as it is a mover, but insofar as it is mobile. For that reason, the definition says that motion is an act of the mobile inasmuch as it is mobile.
303. Then, at “the solution of this difficulty” (202a13), he shows a difficulty that arises from what has been said. For some wonder whether motion is in the mover or in the mobile. But this doubt is solved from what went before. For it is clear that an act of anything is in that thing of which it is the act. Thus, it is plain that the act of motion is in the mobile, since it is an act of the mobile, although caused in it by the mover.
304. Then, at “and the act” (202a15), he shows how motion and mover are related.
He first of all proposes his intention, saying that the act of the mover is not distinct from the act of the mobile. Hence, since motion is an act of the mobile, it is somehow also an act of the mover.
305. Second, at “for it must be” (202a16), he explains this.
And in regard to this, he does three things.
First, he shows that there is an act of the mover as well as of the mobile. For whatever is described according to potency and act has some act corresponding to it. Now, just as that which is moved is called “mobile in potency,” since it is capable of being moved, and is called “moved according to act” inasmuch as it is actually being moved, so a mover is described as a “potential mover” inasmuch as it is able to move, and moves in act inasmuch as it actually acts. Therefore, some act is competent to both, that is, to mover and to mobile.
306. Second, at “but it is on the movable” (202a17), he shows that the act of the mover and that of the mobile are the same act. For something is called mover inasmuch as it acts and moved inasmuch as it is being acted upon.
But what the mover causes by acting and what the moved receives as being acted upon are one and the same thing. And this is what he means when he says that the mover actualizes the mobile, that is, causes the act of the mobile.
Therefore, they must both, the mover and the moved, have the same act, for what is from the mover as agent cause is the same as what is in the moved as patient and receiver.
307. Third, at “just as one to two” (202a18), he illustrates this by an example. For the distance from one to two is the same as that from two to one, but they differ according to account; for in relating two to one, we have double, but in relating one to two, we have half. The same thing is true of the distance covered by one ascending and by one descending, but by reason of the diversity of starting point and term, one is called “ascent” and one “descent.” A parallel case is true of the mover and of the thing moved. For inasmuch as motion proceeds from the mover into the mobile, it is an act of the mover, but inasmuch as it is in the mobile from the mover, it is an act of the mobile.