Commentary Aristotle Physics
Lecture 3. Motion’s definition is suitably formulated
Further, it is evident that motion is an accident of a thing just when it is fully real in this way, and neither before nor after. For each thing of this kind is capable of being at one time actual and at another not.
Take, for instance, the buildable as buildable. The act of the buildable as buildable is the process of building. For the act of the buildable must be either this or the house.
But, when there is a house, the buildable is no longer buildable. On the other hand, it is the buildable that is being built. The process of being built, then, must be the kind of act required.
But building is a kind of motion, and the same account will apply to the other kinds also.
The soundness of this definition is evident both when we consider the accounts of motion that the others have given and also from the difficulty of defining it otherwise. One could not easily put motion and change in another genus.
This is plain if we consider where some people put it. They identify motion with “otherness” or “inequality” or “non-being.” But such things are not necessarily moved, whether they are “other” or “unequal” or “non-existent”; nor is change either to or from these, rather than to or from their opposites.
The reason that they put motion into these genera is that it is thought to be something indefinite, and the principles in the second column are indefinite because they are privative: none of them is either “this” or “such,” or comes under any of the other modes of predication.
In turn, the reason that motion is thought to be indeterminate is that it cannot be classed simply as a potentiality or as an act: a thing that is merely capable of having a certain size is not undergoing change, nor yet a thing that is actually of a certain size; and motion is thought to be a sort of act, but incomplete, and the reason for this view is that the potential whose act it is is incomplete.
This is why it is hard to grasp what motion is. It is necessary to class it with privation or with potentiality or with sheer act, yet none of these seems possible. There remains, then, the suggested mode of definition: that it is a sort of act, or act of the kind described, hard to grasp but not incapable of existing.
291. Having given the definition of motion and an explanation of each of the words in the definition, the Philosopher now shows it to be a good definition:
first, directly;
second, indirectly, at “the soundness of this” (201b16; [293]).
292. In regard to the first (201b5), he uses the following argument: everything that is in potency may at some time be in act, but the buildable is in potency. Therefore, there may at some time be an act of the buildable insofar as it is buildable. But this act is either the house itself or the building of it. But “house” is not the act of the buildable insofar as it is buildable, since the buildable as such is being reduced to act when the building is taking place, but when the house now exists, it is no longer being built. Hence, building is an act of the buildable as such. Building, however, is a certain motion. Motion, therefore, is the act of a thing existing in potency as such. The same is true of other motions. It is clear, therefore, that motion is the type of act described above, and that something is being moved only when it is in such an act, and neither before nor after: not before, since, if it is only in potency, the motion has not begun; nor after, since it has now completely ceased to be in potency by virtue of being in perfect act.
293. Then, at “the soundness of this definition” (201b16), he shows indirectly that it is a good definition by showing that motion cannot be defined in any other way.
In regard to this, he does three things:
first, he proposes what he intends;
second, he presents definitions given by others and rejects them, at “this is plain” (201b19; [294]);
third, he explains why others defined motion as they did, at “the reason that” (201b24; [295]).
He says, therefore, that two things show why the definition given of motion is a good one:
first, because the definitions that others have given are unsuitable;
second, because it is impossible to define motion otherwise than as Aristotle has defined it because motion cannot be placed in any other genus other than that of the act of a thing existing in potency.
294. Then, at “this is plain” (201b19), he excludes the definitions of motion given by others.
These did three things in defining motion.
For they said that motion is otherness because the thing being moved constantly changes from one state to another.
Similarly, they said motion is inequality, because the thing being moved approaches its term always more and more.
They also said that motion is non-being, because the thing being moved does not yet have that to which it is being moved as long as it is being moved—as that which is being moved toward whiteness is not yet white.
These definitions the Philosopher destroys in three ways.
He does so first by looking at the subject of motion. For if motion were otherness, inequality, or non-being, then whatever would possess any one of these three characteristics would of necessity be undergoing motion—in whatever this motion is, that thing is being moved.
But things that are other are not necessarily being moved by the fact that they are other, nor by the fact that they are unequal, nor by the fact that they do not exist. It follows, therefore, that otherness and inequality and non-being are not motion.
Second, he shows the same thing by looking at the term to which the motion is tending, for motion and change do not tend more to otherness than to likeness, or to inequality more than to equality, or to non-being more than to being. For generation is a change to being, and corruption to non-being. Hence, motion is not otherness any more than a likeness, not inequality any more than equality, and not non-being any more than being.
Third, he shows the same thing by looking at the term from which the motion begins, since, just as some motions start from otherness and from inequality and from non-being, so others start from their opposites.
Hence, there is no reason to place motion in the aforementioned genera any more than in their opposites.
295. Then, at “the reason that” (201b24), he points out why some defined motion in the stated ways.
In regard to this, he does two things:
first, he assigns the reason of what has already been stated;
second, he explains a supposition he had made, at “in turn, the reason” (201b27; [296]).
He therefore says first (201b24) that the reason that the older philosophers placed motion in the above-mentioned genera (otherness, inequality and non-being) is that motion seems to be something indeterminate, something incomplete and imperfect, as though possessing no determinate nature. And, because it is indeterminate, its proper place seemed to be in the genus of privation. For when Pythagoras laid down two ordinations of reality, in each of which he placed ten principles, the principles in the second group were said by him to be indeterminate because they were privative. They were not, indeed, determined by a form in the genus of substance, nor by the form of quality or by any special form in either of these genera, or by the form of any of the other predicaments. In one of these groups the Pythagoreans placed ten things: finite, unequal, one, right, male, rest, straight, light, good, and equilateral triangle. In the other, they placed infinite, equal, many, left, female, motion, oblique, dark, evil, and scalene triangle.
296. Then, at “in turn, the reason” (201b27), he gives the reason that motion is placed among the indeterminates. And he says that the reason for this is that motion cannot be placed either in potency or in act.
For if it were placed under potency, whatever would be in potency—for example, to quantity—would be being moved according to quantity. If, on the other hand, it were included under act, then whatever things were actually quantified would be being moved according to quantity.
Now, it is indeed true that motion is act, yet it is imperfect act, a mean between potency and act. And that it is imperfect act is clear from the fact that that of which it is an act is a being in potency as stated above.
And that is why it is difficult to grasp what motion is. For at first sight, it seems to be either entirely act or entirely potency, or else to be contained under privation as it seemed to the ancients who called it non-being or inequality. But none of these is possible, as we have shown above. Hence it follows that there is just one way to define motion: as we have said, it is the act of a thing existing in potency. It is difficult to dwell on such an act on account of the commingling of act and potency; yet that there should be such an act is not impossible, but contingent.