Commentary Aristotle Physics
Book 3. Mobile Being in General
Lecture 1
Natural science treats of motion and those things consequent upon it
Nature is the principle of motion and change, and it is the subject of our inquiry.
We must therefore see that we understand the meaning of motion, for if it were unknown, the meaning of nature would also be unknown.
When we have determined the nature of motion, our next task will be to attack in the same way the terms that are involved in it.
Now, motion is supposed to belong to the class of things that are continuous, and the infinite presents itself first in the continuous—that is how it comes about that “infinite” is often used in definitions of the continuous (“that which is divisible to infinity”). Besides these, place, void, and time are thought to be necessary conditions of motion.
Clearly, then, for these reasons and also because the attributes mentioned are common to and coextensive with all the objects of our science, we must first take each of them in hand and discuss it. For the investigation of special attributes comes after that of the common attributes. Thus, as we said, we begin with motion.
We may start by distinguishing what exists in a state of act only, what exists as potential, and what exists as potential and also in act—one being a “this something,” another quantity, a third quality, and similarly in each of the other modes of the predication of being. Further, the word “relation” is used with reference to excess and defect, agent and patient, and generally what can move and what can be moved. For what can cause motion is relative to what can be moved, and vice versa.
Again, there is no such thing as motion outside of things. It is always with respect to substance or to quantity or to quality or to place that what changes changes. But it is impossible, as we assert, to find anything common to these that is neither “this” nor quantity nor quality nor any of the other predicates. Hence, neither will motion and change have reference to something over and above the things mentioned, for there is nothing over and above them.
Now, each of these belongs to all its subjects in either of two ways: in substance, as either its form or its privation; in quality, such as either white or black; in quantity, as complete and incomplete; in respect of locomotion, as upward and downward or light and heavy. Hence, there are as many types of motion or change as there are meanings of the word “is.”
275. Having settled the question of the principles of natural things, and that of the principles of this science, the Philosopher here begins to pursue his original plan, which is to arrive at conclusions concerning the subject of this science, mobile being taken absolutely.
The treatment, then, is divided into two parts:
in the first, he comes to a conclusion about motion in itself;
in the second, he comes to a conclusion about movers and things movable, in book 7, at “everything that is in motion” (241b24; [884]).
The first part is divided into two:
in the first, he comes to a conclusion about motion itself;
in the second, he comes to a conclusion about its parts, in book 5, at “everything that changes” (224a21; [638]).
As to the first, he does two things:
first, he states what is under investigation;
then he explains it, at “we may start” (200b26; [279]).
With reference to the first of these, he does two things:
first, he says what he intends to principally treat;
second, he sets down certain things connected with it, with which he will be subsequently concerned, at “when we have determined” (200b15; [277]).
276. As to the first (200b12), he uses the following argument. Nature is the principle of motion and change, as is evident from the definition set down in book 2 (though how motion and change differ will be shown in book 5).
And thus it is evident that, if one does not know motion, one does not know nature, since the former is placed in the definition of the latter. Therefore, since we intend to present the science of nature, we must make motion understood.
277. Then, at “when we have determined” (200b15), he adds certain things that accompany motion. And he employs two sets of reasons for including them.
The first is as follows: whoever determines something must determine those things that follow upon it, for the subject and its accidents are considered in a single science.
But the infinite follows upon motion intrinsically, as the following makes plain: motion is within the number of continuous things, as will be evident below in book 6. But “infinite” enters into the definition of “continuum.”
And he adds first because the infinite that is found in the addition of number is caused from the infinite that is in the division of the continuum.
And that the infinite enters into the definition of the continuum he shows from the fact that those defining the continuum often use “infinite”—as, for example, when they say that the continuum is that which is divisible to infinity.
And he says often because there is also found another definition of the continuum, which is given in the Categories: the continuum is that whose parts are joined at a common boundary.
Now, these two definitions differ. For the continuum, since it is a certain whole, is properly defined through its parts. But parts are compared to the whole in a twofold way: according to composition, insofar as the whole is composed out of the parts; and according to resolution, insofar as the whole is divided into the parts.
The present definition of the continuum is therefore given according to the mode of resolution, while that which is set down in the Categories is according to the mode of composition. Hence it is clear that the infinite follows upon motion intrinsically.
But there are some things that follow upon motion extrinsically, as certain external measures, such as place, the void, and time. For time is the measure of motion itself, while the measure of the mobile thing is indeed place according to truth, but the void according to the opinion of some. And therefore he adds that motion cannot be without place, the void, and time. Nor does the fact that not all motion is local affect this, since nothing is moved that is not in place. For every sensible body is in place, and to sensible body alone does it belong to be moved.
Likewise, local motion is the first of motions, and when it is removed, the other motions are removed, as will be evident below in book 8. It is thus clear that the four above-mentioned properties are consequent upon motion and, hence, pertain to the consideration of the natural philosopher for the given reason.
278. This is also true for yet another reason that he adds subsequently: because the properties just stated are common to all natural things.
Accordingly, since it is the task of natural science to reach conclusions concerning all natural things, one must therefore first determine concerning each of these.
For the speculation that is directed toward proper things comes after that which is of common things, as was stated in the beginning. But, among all these common things, one must first reach conclusions concerning motion itself, because the other things follow upon it, as was stated.
279. Then, at “we may start” (200b26), he puts his plan into execution.
And first, he reaches conclusions concerning motion and the infinite, which follows motion intrinsically;
second, he does the same in book 4 for the other three, which follow motion extrinsically, at “the physicist must have a knowledge” (208a27; [406]).
The first treatment is divided into two parts:
first, he comes to a conclusion about motion;
second, he does the same for the infinite, at “the science of nature is concerned” (202b30; [326]).
With respect to the first of these, he does two things:
first, he prefaces his treatment with certain considerations requisite for investigating the definition of motion;
second, he defines motion, at “we have now” (201a9; [283]).
As to the first of these, he does two things:
first, he sets down in advance certain divisions, since the most suitable path toward finding definitions is through division, as the Philosopher shows in Posterior Analytics 2.12 and in Metaphysics 7;
second, he shows that motion falls within the given divisions, at “again, there is no such thing” (200b32; [281]).
280. With respect to the first of these (200b26), he sets down three divisions.
The first of these is that being is divided by potency and act. Now, this division does not distinguish beings into genera—for potency and act are found in every genus.
The second division is of being as divided according to the ten genera. The first of these is this something, namely, substance; others are quantity or quality or some other of the predicaments.
The third division is of one genus of beings, namely, of the one which is relation. For motion seems in a certain way to pertain to this genus, insofar as the mover is referred to the movable thing.
In order to understand this third division, one must consider that, since relation has the weakest existence—consisting alone, as it does, in the fact of being something referred to something else—it is necessary that it be grounded on some other accident.
For the more perfect accidents are closer to the substance, and it is through them as intermediates that the other accidents inhere in the substance.
Now, relation is founded chiefly upon two accidents that have an order to something else: upon quantity and action. For quantity may be a measure even of something external to it; but the agent transfuses its action into something other than itself.
Accordingly, certain relations are founded upon quantity, and especially upon that species of quantity which is number, to which the basic account of measure pertains—as is evident in “double and half,” “multiple and fraction,” and other such things.
Similarly, “same,” “like,” and “equal” are founded upon unity, which is the principle of number.
Still other relations are founded upon action and passion: either according to existing act, as something is said to be heating in relation to that which is heated; or according to having acted, as a father is referred to a son because he engendered him; or else according to the possibility of acting, as a master is related to a servant because he is able to make him do something.
Now, the Philosopher clearly explains this division in Metaphysics 5, but he here touches on it briefly, saying that one sort of relation is that according to excess and defect, which sort, indeed, is founded on quantity, as in the case of double and half; while the other is according to active and passive, and mover and movable, which are referred to each other, as is self-evident.
281. Then, at “again, there is no such thing” (200b32), he shows how motion is reduced to the given divisions.
And as to this, he does two things:
first, he shows that motion is not outside the genera of things in which motion occurs;
second, he shows that motion is divided as the genera of things are divided, at “now, each of these” (201a3; [282]).
As to the first of these, it should be observed that since motion is an imperfect act, as will be evident below, and since everything that is imperfect falls under the same genus with that which is perfect—not, indeed, as a species, but by reduction (as prime matter is by reduction in the genus of substance)—motion is necessarily not outside the genera of things in which motion occurs.
And this is what he states, that motion is not outside of things—that is, outside the genera of things in which motion is found—in such a way as to be something extraneous to, or something common to, these genera. And he makes this plain by the fact that whatever is changed is changed according to substance, quantity, quality, or place, as will be shown in book 5.
Now, there is not some common univocal element in these genera that would not be found under some predicament but would be their genus; but being is common to them according to analogy, as will be shown in Metaphysics 4. Hence, it is also plain that neither motion nor change is outside the given genera, since nothing is outside the latter and they sufficiently divide being. But, as to the question of how motion is related to the predicament of action or passion, this will be explained below.
282. Then, at “now, each of these” (201a3), he shows that motion is divided as the genera of things are divided. For it is plain that, in all the genera, a thing may be present in two ways: either as something perfect or as something imperfect. The reason for this is that privation versus possession is the prime contrariety, which is found in all the contraries, as is stated in Metaphysics 10.
Hence, since all the genera are divided through contrary differences, it is necessary that in all there be the perfect and the imperfect: in substance, something is as form and something is as privation; in quality, there is something such as white, which is perfect, and something such as black, which is, as it were, imperfect; in quantity, one thing is perfect quantity and another imperfect; and in place, something is above, which is, as it were, perfect, and something is below, which is, so to speak, imperfect; or else there is light and heavy, which are placed in place by virtue of the inclination that is in them.
Hence, it is plain that, according to the divisions of being, there are corresponding divisions of motion. For the species of motion differ according to the different genera of being. Thus, increase, which is motion in quantity, differs from generation, which is motion in substance. The species of motion likewise differ according to perfect and imperfect in the same genus: generation is motion in substance toward form, while corruption is motion toward privation; and in quantity, increase is toward perfect quantity, diminution toward imperfect. But, as to the question of why there are not assigned two kinds in quality and place, this will be explained in book 5.