LESSON 9

The Definition of Motion

  Chapter 9: 1065b 5-1066a 34

             974. One thing is actual only, another potential, and others both actual and potential; and 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, for example, 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.

             975. Now since each class of things is divided by potentiality and actuality, I call motion the actualization of what is potential as such.

             976. 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.

             977. And by the phrase inasmuch as I mean this: bronze is potentially a statue, but the actuality of bronze inasmuch as it is bronze is not motion; for to be bronze and to be some potentiality are not the same. If they were absolutely the same in meaning, the actuality of bronze would be a kind of motion; but they are not the same. This is evident in the case of contraries; for the potentiality 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, whether it be moisture or blood, that is one and the same. 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.

             978. 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 actuality 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.

             979. 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 class.

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

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

             982. The reason for putting motion in this class is that it seems to be something indefinite; and the principles in one of the columns of opposites (60) are indefinite because they are privative, for no one of them is either a this or such or any of the other categories.

             983. The reason why motion seems to be indefinite is that it cannot be identified either with the potentiality or with the actuality 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 actuality, but an incomplete one; and the reason for this is that the potentiality of which it is the actuality is incomplete. Hence it is difficult to grasp what motion is; for it must be put under privation or under potentiality or under simple actuality; but none of these appear to be possible. It remains, then, that it must be as we have said, i.e., both an actuality and a non-actuality as explained, which is difficult to see but capable of existing.

             984. 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.

             985. And the actuality of what is capable of causing motion is no other than this; for it must be the actuality of both.

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

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

             988. 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.

COMMENTARY

             2289. Having settled the issue about accidental being, the Philosopher now states his views about motion; and this is divided into three parts. First (974:C 2289), he deals with motion in itself; second (989:C 2314), with infinity, which a property of motion and of other continuous things ("The infinite"); and third (1005:C 2355), with the division of motion into its species ("Everything which is changed").

             The first is divided into two parts. First, he explains what motion is; and second (984:C 2308), he points out what the subject of motion is ("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 (975:C 2294), he defines motion ("Now since each"). Third (979:C 2299), he proves that the definition of motion is a good one ("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 actuality and by potentiality. 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, 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, i.e., 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 actuality 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 actuality. 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; and in the genus of quality there is one thing which is perfect, as a white thing, which has a perfect color, and another which is imperfect, as 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"; and in the genus of place, in which "motion in space" is found, i.e., local motion, one thing tends upwards and another downwards, 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 the character 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 actuality and potentiality 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 (975).

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

             2295. That our account (976).

             Second, he explains the definition which has been given; and in regard to this he does two things. First (976:C 2295), he explains what was given in the definition with regard to the subject of motion; and second (978:C 2297), what was given as the genus of motion ("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; and second (977:C 2296), the part, insofar as it is such ("And by the phrase").

             He accordingly says, first (976), 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 potentiality, and that the potentiality in question is presented as being brought to actuality by what is designated as being built; and this actuality 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 potentiality when it is being brought to actuality; and by this I mean that it is being brought to actuality insofar as it is movable; for a thing is said to be movable because it is in potentiality to motion. Hence a potentiality of this kind is being brought to actuality when it is actually being moved; but what is potential "inasmuch as it is itself," i.e., in reference to what it actually is and in itself, does not have to be brought to actuality by motion. For it actually is this before it begins to be moved. And neither is it being brought to actuality by motion insofar as it is in potentiality to the terminus of motion, because so long as it is being moved it still remains in potentiality to the terminus of motion. But a thing is being brought from potentiality to actuality by motion only in the case of that potentiality which is signified when a thing is said to be movable, i.e., capable of being moved.

             2296. And by the phrase (977).

             Then he explains a phrase which was given in the definition of motion, namely, insofar 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 potentiality to being a statue, and thus the subject bronze and bronze in potentiality 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 potentiality are different; and this is what he means when he says that to be bronze and to be some potentiality are not the same. For if they were the same in their meaning, then just as motion is an actuality of bronze insofar as it is bronze in potentiality, in a similar way motion would be the actuality of bronze insofar as it is bronze. But bronze and the potentiality of bronze do not have the same meaning. This is evident in the case of the potentiality for contraries, because the potentiality "of being healed and that of being ill" do not have the same meaning; for the concept of a potentiality is derived from that of the actuality. Hence, if the potentiality 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 potentiality 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 potentiality 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 which are essentially the same as some third thing are themselves essentially the same. Hence, since bronze and bronze in potentiality 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 (978).

             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 actuality 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 potentiality and at another time is in a state of actualization. He says "the buildable as buildable" because the matter of a house is in potentiality 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 potentiality to both and at another to be in a state of actuality. But the potentiality 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 actuality of the buildable as buildable.

             2298. He proves this as follows: the matter of a house is in potentiality to only two actualities, namely, the act of building the house and the form of the house. But the term buildable signifies a potentiality belonging to the matter of the house. Therefore, since there is some actuality corresponding to every potentiality, the potentiality signified by the term buildable must correspond to one of these two actualities, namely, either to the form of the house or to the act of building. But the form of the house is not the actuality 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 actuality when the house is actually being built. Therefore the act of building must be the actuality of the buildable. Now the act of building is a kind of motion; and thus motion is the actuality of the buildable. The same explanation holds for all other motions. It is evident, then, that motion is the actuality of what is potential.

             2299. That this account (979).

             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 class than in that of actuality.

             2300. This is evident (980).

             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 (981).

             Third, he shows that the definitions given above are not suitable ones; for they do not fit motion so far as its subject is concerned, i.e., the thing moved. 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 nonbeing to being and vice versa, and from otherness to likeness, and from inequality to equality and vice versa.

             2302. The reason (982).

             Fourth, he shows why some defined motion in the foregoing way. He says that the reason why they put motion in the above-mentioned class is that motion seems to be something indefinite, and things which 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 I (60:C 127) of this work, that the Pythagoreans posited two orders of things, and in one of these, which they called the order of good things, they placed things which seem to be perfect, for example, light, right, male, rest, and the like; and 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," i.e., substance, "or such," i.e., quality, or any of the other categories.

             2304. The reason why (983).

             Fifth, he points out why motion is placed in the class of the indefinite. The reason for this, he says, is that motion can be placed neither in the class of the potential nor in that of the actual; for if it were placed in the class of the potential, it would follow that whatever is in potentiality to something, for example, to have 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 potentiality to that quantity. Moreover, it is not being moved when it already actually has that quantity to which it was in potentiality, but the motion has then already been terminated.

             2305. But motion must be a kind of actuality, as has been proved above (975:C 2294), although it is an imperfect one. The reason for this is that the thing of which it is the actuality is imperfect, and this is a possible or potential being; for if it were a perfect actuality, the whole potentiality for some definite actuality which is in the matter would be eliminated. Hence perfect actualities are not actualities of something in potentiality but of something in actuality. But motion belongs to something that is in potentiality, because it does not eliminate the potentiality of that thing. For so long as there is motion, the potentiality for that to which it tends by its motion remains in the thing moved. But only the previous potentiality for being moved is eliminated, though not completely; for what is being moved is still in potentiality to motion, because everything which is being moved will be moved, because of the division of continuous motion, as is proved in Book VI of the Physics. It follows, then, that motion is the actuality of what is potential; and thus it is an imperfect actuality and the actuality 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 class of privation, as is evident from the definitions given above, or in the class of potentiality, or in that of simple and complete actuality--none of which may be moved. It follows, then, that motion is as we have described it to be, namely, an actuality, and that it is not called a perfect actuality. This is difficult to grasp, although 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 potentiality to actuality. But they erred, because 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 (984).

             Then he explains what the subject of motion is. First, he shows that it is the thing moved; because every actuality is found in the thing whose actuality it is. But motion is the actuality 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 actuality (985).

             Second, he shows how motion is related to a mover; and he gives two points, namely, that motion is the actuality of what is capable of causing motion, and that the actuality of the thing capable of causing motion and that of the thing moved do not differ; for motion must be the actuality of both.

             2310. And a thing is capable (986).

             Third, he proves the first of these two points, namely, that motion is the actuality of what is capable of causing motion. For the actuality 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, and it is said to be a mover because of its activity, i.e., because it is actual. Hence, since a thing is said to be a mover because of motion, motion will be the actuality of what is capable of causing motion.

             2311. But it is (987).

             Fourth, he proves the second of these points, namely, that the actuality of what is capable of causing motion and the actuality 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 actuality 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 actuality of the mover and that of the thing moved are one and the same motion.

             2312. And it is one (988).

             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 actuality of both is essentially one motion, although they differ conceptually. For the actuality of a mover functions as that from which motion comes, whereas the actuality of the thing moved functions as that in which motion occurs. And the actuality of the thing moved is not that from which motion comes, nor is the actuality of the mover that in which motion occurs. Hence the actuality of the thing causing motion is called action, and that of the thing moved is called undergoing or suffering.

             2313. But if action and undergoing 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; and 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 where. Similarly, inasmuch as motion is predicated of the subject in which it is found, it constitutes the category of undergoing; but inasmuch as it is predicated of that from which it comes, it constitutes the category of action.