Lecture 9 — The definition of motion

One thing is actual only, another potential only, and others both actual and potential. Of these, one is a being, another a quantity, and another one of the other categories. Motion is not something apart from things themselves, for a thing is always changed according to the categories of being, and there is nothing that is common to these and in no one category.

And each belongs to all its members in a twofold way—such as this particular thing—for sometimes this is the form of a thing and sometimes its privation.

And with regard to quality, one thing is white and another black; and with regard to quantity, one is perfect and another imperfect; and with regard to motion in space, one thing tends upwards and another downwards, or one is light and another heavy.

Hence there are as many kinds of motion and change as there are of being.

Now since each genus of things is divided by potency and act, I call motion the actualization of what is potential inasmuch as it is such.

That our account is true becomes evident as follows. When the buildable in the sense in which we call it such actually exists, it is being built, and this is the process of building. The same is true of learning, walking, healing, dancing, and bereaving. And motion occurs when something is in this very act, and neither before nor after. Motion, then, belongs to what is potential when it is actual and is engaged in activity, not inasmuch as it is itself, but inasmuch as it is movable.

And by the phrase “inasmuch as,” I mean this. Bronze is potentially a statue, but the act of bronze inasmuch as it is bronze is not motion, for to be bronze and to be some potency are not the same. If they were absolutely the same in meaning, the act of bronze would be a kind of motion, but they are not the same.

This is evident in the case of contraries, for the potency of being healed and that of being ill are not the same, because being healed would then be the same as being ill. But it is the subject which is both healed and ill that is one and the same, whether it be moisture or blood. And since they are not the same, just as color and the visible object are not the same, it is the actualization of what is potential insofar as it is potential that is motion.

That motion is this, and that a thing is being moved when it is actual in this way, and neither before nor after, is evident. For each thing is capable of being at one time actual and at another not—for example, the buildable as buildable. And the actualization of the buildable as buildable is the process of building. For the act is either the process of building or this particular house. But when the house exists, it will no longer be buildable; but what is being built is what is buildable. Therefore, the process of building must be its actualization, and the process of building is a kind of motion. The same reasoning also applies to other motions.

That this account is true is evident from what others say about motion, and because it is not easy to define it otherwise. For one cannot place it in another genus.

This is evident from what some say, for they call it otherness and inequality and non-being.

However, no one of these is necessarily moved, and change is not to these or from these anymore than to or from their opposites.

The reason for putting motion here is that it seems to be something indefinite, and the principles in one of the columns of opposites (986a22) are indefinite because they are privative, for no one of them is either a this or such or any of the other categories.

The reason why motion seems to be indefinite is that it cannot be identified either with the potency or with the act of existing things, for neither what is capable of having a certain quantity nor what actually has it is necessarily being moved. And motion seems to be an act, but an incomplete one. The reason for this is that the potency of which it is the act is incomplete. Hence, it is difficult to grasp what motion is, for it must be put under privation or under potency or under simple act, but none of these appear to be possible. It remains, then, that it must be as we have said, namely, both an act and a non-act, as explained, which is difficult to see, but capable of existing.

That motion belongs to the thing moved is evident, for it is the actualization of the thing moved by what is capable of causing motion.

And the act of what is capable of causing motion is no other than this, for it must be the act of both.

And a thing is capable of causing motion because of its power, but it is a mover because of its activity.

But it is on the thing moved that it is capable of acting. Thus, the act of both alike is one.

And it is one just as the distance from one to two and that from two to one are the same, and just as what goes up and what comes down are the same, although their being is not one. The same applies in the case of the mover and the thing moved.

2289. Having settled the issue about accidental being, the Philosopher now states his views about motion, and this is divided into three parts.

First (1065b5; [2289]), he deals with motion in itself;

second (1066a35; [2314]), with infinity, which is a property of motion and of other continuous things, at the infinite;

third (1067b1; [2355]), with the division of motion into its species, at either everything which is changed.

The first is divided into two parts.

First, he explains what motion is;

second (1066a26; [2308]), he points out what the subject of motion is, at that motion.

In regard to the first, he does three things.

First, he prefaces his discussion with some points which are necessary for defining motion.

Second (1065b14; [2294]), he defines motion, at now since each.

Third (1066a7; [2299]), he proves that the definition of motion is a good one, at that this account.

In treating the first member of this division, he gives four points, from which he infers a fifth.

The first is that being is divided by act and by potency. He says that one kind of being is actual only, such as the prime mover, which is God; another is potential only, such as prime matter; and others are both potential and actual, such as all intermediate things.

Or by the phrase actual only, he means what already has a form completely, as what is now completely white; and by potential only, what does not have a form, as what is not white in any way; and by potential and actual, what does not yet have a form completely but is being moved to a form.

2290. The second point is that being is divided by the ten categories, as is implied when he says that there is one kind of being which exists of itself, namely, substance, and another is quantity, and another is quality, and so on for the other categories.

2291. The third point is that motion does not have a distinct nature separate from other things, but every form insofar as it is in a state of becoming is an imperfect act, which is called motion. For to be moved to whiteness is the same as for whiteness to begin to become actual in a subject, but it need not be in complete act. This is his meaning in saying that motion is not something apart from things themselves, for everything which is being changed is being changed according to the categories of being. And just as the ten categories have nothing in common as their genus, in a similar way, there is no genus common to all the kinds of motion. Hence motion is not a category distinct from the others, but is a natural concomitant of the other categories.

2292. The fourth point is that a thing is found in any genus in two ways, namely, perfectly and imperfectly. For example, in the genus of substance, one thing has the character of a form and another the character of a privation.

Likewise, in the genus of quality, there is one thing which is perfect, like a white thing, which has a perfect color, and another which is imperfect, like a black thing, which is imperfect in the genus of color.

And in the genus of quantity, one thing is perfect, and this is called “great,” and another is imperfect, and this is called “small.”

In the genus of place, where motion in space is found (that is, local motion), one thing tends upward and another downward, or one is light and another heavy inasmuch as that is called “light” which actually rises upwards and that “heavy” which actually sinks downwards; and one of these has the character of something perfect and the other of something imperfect.

The reason is that all the categories are divided by contrary differences, and one contrary always has the character of something perfect, and the other the character of something imperfect.

2293. From these four points, he infers a fifth, namely, that there are as many kinds of motion and change as there are of being. He does not say this because there is motion in every genus of being, but because, just as being is divided by act and potency and by substance and accident and the like, and in terms of perfect and imperfect, so also is motion. This follows from his assertion that motion is not something apart from things. The way in which change and motion differ will be explained below.

2294. Now since each (1065b14). Next, he defines motion.

First, he gives its definition, saying that, since in each genus of being, being is divided by potency and act, motion is said to be the actualization of what is potential insofar as it is such.

2295. That our account (1065b16). Second, he explains the definition which has been given, and in regard to this, he does two things.

First (1065b16; [2295]), he explains what was given in the definition with regard to the subject of motion;

second (1065b34; [2297]), what was given as the genus of motion, at that motion is this.

In regard to the first member of this division, he does two things.

First, he explains the part of the definition “what is potential”;

second (1065b23; [2296]), the part “inasmuch as,” at and by the phrase.

He accordingly first says (1065b16) that it is evidently true from this that motion is as we have described it to be. For it is clear that the term “buildable” signifies something in potency, and that the potency in question is presented as being brought to act by what is designated as being built. This act is called the process of building. The same thing is also true of other motions, such as walking, altering, and the like. And a thing is said to be being moved when it is coming to be such and such actually and has been such and such potentially, and neither before nor after. If this is so, then, it follows that motion belongs to a thing in potency when it is being brought to act. By this, I mean that it is being brought to act insofar as it is movable, for a thing is said to be movable because it is in potency to motion.

Hence a potency of this kind is being brought to act when it is actually being moved; but what is potential inasmuch as it is itself, that is, in reference to what it actually is and in itself, does not have to be brought to act by motion. For it actually is this before it begins to be moved. And neither is it being brought to act by motion insofar as it is in potency to the terminus of motion, for so long as it is being moved, it still remains in potency to the terminus of motion. But a thing is being brought from potency to act by motion only in the case of that potency which is signified when a thing is said to be movable, that is, capable of being moved.

2296. And by the phrase (1065b23). Then he explains a phrase which was given in the definition of motion, namely, “inasmuch as it is such,” or “inasmuch as it is of this kind.” With a view to making this clear, he says that bronze is in potency to being a statue, and thus the subject bronze and bronze in potency to being a statue are the same, although they are not the same in their meaning. For the concept of bronze as bronze and that of bronze insofar as it has some potency are different, and this is what he means when he says that to be bronze and to be some potency are not the same.

For if they were the same in their meaning, then, just as motion is an act of bronze insofar as it is bronze in potency, in a similar way, motion would be the act of bronze insofar as it is bronze. But bronze and the potency of bronze do not have the same meaning. This is evident in the case of the potency for contraries, because the potency of being healed and that of being ill are not the same conceptually, for the concept of a potency is derived from that of the act.

Hence, if the potency of being healed and that of being ill were the same in meaning, it would follow that being healed and being ill are the same. But this is impossible. Therefore, the potency for each of two contraries is not the same in meaning, although it is the same in subject. For it is the same subject which can be healed or be ill; and whether that subject is any one at all of the humors in the animal’s body, or the blood (which is more natural and proper to the life and nourishment of the animal), it can be a cause of its being healed or being ill.

Since, then, the potency of being healed and that of being ill are not the same in meaning, it is evident that neither of these is the same as its subject in meaning, because any two things that are per se the same as some third thing are themselves per se the same. Hence, since bronze and bronze in potency to being a statue are not the same in meaning, just as color and the visible object are not the same, the phrase “insofar as it is such” must be added to the statement that motion is the actualization of what is potential.

2297. That motion is this (1065b34). Then he explains the term which was given as the genus in the definition of motion. That motion is this is evident, he says, because the said motion then exists when it (the act of what is potential) is actual in this way, and neither before nor after. For, obviously, every movable thing can be at one time in a state of actualization and at another not, for the buildable as buildable at one time is in a state of potency and at another time is in a state of actualization. He says the buildable as buildable because the matter of a house is in potency to two things, namely, to the form of a house, and to the process of being built. And it is possible for it at one time to be in a state of potency to both and at another to be in a state of act. But the potency which the matter of a house has for being built is signified by the term “buildable.” Therefore, the buildable as buildable becomes actual when it is being built, and thus the process of building is the act of the buildable as buildable.

2298. He proves this as follows. The matter of a house is in potency to only two acts, namely, the act of building the house and the form of the house. But the term “buildable” signifies a potency belonging to the matter of the house. Therefore, since there is some act corresponding to every potency, the potency signified by the term “buildable” must correspond to one of these two acts, namely, either to the form of the house or to the act of building. But the form of the house is not the act of the buildable as buildable, because when the form of the house develops, the house is no longer buildable, but is already built. But the buildable is in a state of act when the house is actually being built. Therefore, the act of building must be the act of the buildable. Now, the act of building is a kind of motion, and thus motion is the act of the buildable. The same explanation holds for all other motions. It is evident, then, that motion is the act of what is potential.

2299. That this account (1066a7). Then he proves that the definition given is a good one.

First, he gives a general proof. He says that it is evident that this definition of motion is a good one if we consider what others have said about motion when they defined it, and also because it cannot easily be defined in a different way. For it cannot be put in any other genus than in that of act.

2300. This is evident (1066a10). Second, he states what others have said about motion. He says that some have said that motion is otherness, others inequality, and others non-being. And perhaps they spoke of it thus because the thing being moved gradually loses its initial state, and so long as it is being moved, it is always in different states and comes closer to its goal.

2301. However, no one (1066a11). Third, he shows that the definitions given above are not suitable ones, for they do not fit motion so far as its subject (the thing moved) is concerned. For if motion were non-being or inequality or otherness, it would follow that every non-being or whatever is other or unequal is moved, but it is not necessary that any of these should be moved. Hence, motion is not as they have described it to be. The same thing is also apparent with regard to the termini of motion, which are the limits from which and to which there is motion. For motion is not to non-being or inequality or otherness rather than to their opposites, nor is motion from these rather than from their opposites. For there can be motion from non-being to being and vice versa, and from otherness to likeness, and from inequality to equality and vice versa.

2302. The reason (1066a13). Fourth, he shows why some defined motion in the foregoing way. He says that the reason why they put motion in the above-mentioned genus is that motion seems to be something indefinite, and things that are privative are indefinite. Hence they assumed that motion is a kind of privation.

2303. It should also be noted, as has been pointed out in book 1 (986a22; [127]) of this work, that the Pythagoreans posited two orders of things.

In one of these, which they called the order of good things, they placed things that seem to be perfect, such as light, right, male, rest, and the like.

In the other order, which they listed under evil, they placed darkness, left, female, motion, and the like. And they said that all such things are indefinite and privative because no one of them seems to signify either a this, that is, substance, or such, that is, quality, or any of the other categories.

2304. The reason why (1066a17). Fifth, he points out why motion is placed in the genus of the indefinite. The reason for this, he says, is that motion can be placed neither in the genus of the potential nor in that of the actual; for, if it were placed in the genus of the potential, it would follow that whatever is in potency to something—such as having some quantity—would be moved to that quantity. But this is not necessary, because, before a thing begins to be moved to some quantity, it is in potency to that quantity. Moreover, it is not being moved when it already actually has that quantity to which it was in potency, but the motion has then already been terminated.

2305. But motion must be a kind of act, as has been proved above (1065b14; [2294]), although it is an imperfect one. The reason for this is that the thing of which it is the act is imperfect, and this is a possible or potential being, for if it were a perfect act, the whole potency for some definite act which is in the matter would be eliminated.

Hence, perfect acts are not acts of something in potency, but of something in act. But motion belongs to something that is in potency, because it does not eliminate the potency of that thing. For so long as there is motion, the potency for that to which it tends by its motion remains in the thing moved. But only the previous potency for being moved is eliminated, though not completely, for what is being moved is still in potency to motion, for everything which is being moved will be moved because of the division of continuous motion, as is proved in Physics 6. It follows, then, that motion is the act of what is potential; thus it is an imperfect act and the act of something imperfect.

2306. It is because of this that it is difficult to grasp what motion is, for it seems necessary to place motion either in the genus of privation, as is evident from the definitions given above, or in the genus of potency, or in that of simple and complete act—none of which may be moved. It follows, then, that motion is as we have described it to be, namely, an act, and that it is not called a perfect act. This is difficult to grasp, but it can nevertheless be true, because, when this is admitted, nothing untenable follows.

2307. Some have defined motion by saying that it is the gradual passage from potency to act. But they erred, because [it is rather that] motion must be given in the definition of a passage, since it is a kind of motion. Similarly, time is placed in the definition of the gradual, and motion in the definition of time.

2308. That motion belongs (1066a26). Then he explains what the subject of motion is.

First, he shows that it is the thing moved, because every act is found in the thing whose act it is. But motion is the act of the movable by what is capable of causing motion. Hence it follows that motion is found in the movable or thing moved, and that it is the actualization of this is clear from the above discussion.

2309. And the act (1066a28). Second, he shows how motion is related to a mover, and he gives two points: that motion is the act of what is capable of causing motion, and that the act of the thing capable of causing motion and that of the thing moved do not differ (for motion must be the act of both).

2310. And a thing is capable (1066a29). Third, he proves the first of these two points, namely, that motion is the act of what is capable of causing motion. For the act of a thing is that by which it becomes actual. But a thing is said to be capable of causing motion because of its power of moving, but it is a mover because of its activity, that is, because it is actual. Hence, since a thing is said to be a mover because of motion, motion will be the act of what is capable of causing motion.

2311. But it is (1066a30). Fourth, he proves the second of these points—namely, that the act of what is capable of causing motion and the act of what is capable of being moved are one and the same motion. He does this as follows.

It has been stated that motion is the act of what is capable of causing motion inasmuch as it causes motion, and a thing is said to be movable inasmuch as motion is caused in it, but the thing capable of causing motion causes that motion which is found in the thing moved and not a different one.

This is what he means when he says that it is on what is movable that the mover is capable of acting. It follows, then, that the act of the mover and that of the thing moved are one and the same motion.

2312. And it is one (1066a31). Fifth, he clarifies this by an example.

He says that the distance from one to two and from two to one are the same, although they differ conceptually, and for this reason the distance is signified differently, namely, by the terms double and half.

Similarly, the path of an ascent and that of a descent are one, but they differ conceptually, and for this reason some are called ascenders and others descenders.

The same applies to a mover and to the thing moved, for the act of both is essentially one motion, although they differ conceptually.

For the act of a mover is as that from which motion comes, whereas the act of the thing moved is as that in which motion occurs; and the act of the thing moved is not from which, nor is the act of the mover that in which motion occurs.

Hence, the act of the thing causing motion is called action, and that of the thing moved is called passion.

2313. But, if action and passion are essentially the same thing, it seems that they should not be different categories. However, it should be borne in mind that the categories are distinguished on the basis of a different way of predicating. Thus, inasmuch as the same term is differently predicated of different things, it belongs to different categories; for inasmuch as place is predicated of a thing that locates, it belongs to the genus of quantity, but inasmuch as it is predicated denominatively of the located thing it constitutes the category of where. Similarly, inasmuch as motion is predicated of the subject in which it is found, it constitutes the category of passion; but inasmuch as it is predicated of that from which it comes, it constitutes the category of action.