Commentary Aristotle Physics
Lecture 2. Arguments for the eternity of motion
Let us take our start from what we have already laid down in our course on physics. Motion, we say, is the act of the movable insofar as it is movable. Each kind of motion, therefore, necessarily involves the presence of the things that are capable of that motion.
In fact, even apart from the definition of motion, everyone would admit that, in each kind of motion, it is that which is capable of that motion that is in motion, and thus it is that which is capable of alteration that is altered, and that which is capable of local change that is in locomotion.
And, likewise, there must be something combustible before there can be a being burned, and something combustive before there can be a burning.
Moreover, these things also must either have a beginning before which they had no being, or they must be eternal. Now, if there was a becoming of every movable thing, it follows that, before the motion in question, another change or motion must have taken place in which that which was capable of being moved or of causing motion had its becoming.
To suppose, on the other hand, that these things were in being throughout all previous time without there being any motion appears unreasonable on a moment’s thought, and still more unreasonable, we shall find, on further consideration.
For if, while there are things that are movable and things that are motive, there is a time when there is a first mover and a first moved and another time when there is no such thing, but only something that is at rest, then what is at rest must previously have been in process of change.
And there must have been some cause of its rest, rest being the privation of motion. Therefore, before this first change, there will be a previous change.
For some things cause motion singularly, while others can produce either of two contrary motions: fire causes heating but not cooling, while it would seem that knowledge may be directed to two contrary ends while remaining one and the same.
Even in the former class, however, there seems to be something similar, for a cold thing causes heating, in a sense, by turning away and retiring, just as one possessed of knowledge voluntarily makes an error when he uses his knowledge in the reverse way.
But, at any rate, all things that are capable respectively of affecting and being affected, or of causing motion and being moved, are capable of it not under all conditions, but only when they are in a particular condition and approach one another.
It is on the approach of one thing to another that the one causes motion and the other is moved, and when they are present under such conditions as rendered the one motive and the other movable.
So, if the motion was not always in process, it is clear that they must have been in a condition not such as to render them capable respectively of being moved and of causing motion, and one or other of them must have been in process of change.
For in what is relative, this is a necessary consequence. For instance, if one thing is double another when before it was not so, one or other of them (if not both) must have been in process of change. It follows, then, that there will be a process of change previous to the first.
Further, how can there be any “before” and “after” without the existence of time? Or how can there be any time without the existence of motion?
If, then, time is the number of motion or itself a kind of motion, it follows that, if there is always time, motion must also be eternal.
But, so far as time is concerned, we see that all, with one exception, are in agreement in saying that it is uncreated.
In fact, it is just this that enables Democritus to show that all things cannot have had a generation, for time, he says, is not begotten.
Plato alone generates time, saying that it had a generation together with the universe, the universe according to him having had a generation.
Now, since time cannot exist and is unthinkable apart from the moment, and the moment is a kind of middle point, uniting as it does in itself both a beginning and an end—a beginning of future time and an end of past time—it follows that there must always be time.
For the extremity of the last period of time that we take must be found in some moment, since time contains no point of contact for us except the moment.
Therefore, since the moment is both a beginning and an end, there must always be time on both sides of it. But, if this is true of time, it is evident that it must also be true of motion, time being a kind of passion of motion.
The same reasoning will also serve to show that motion is incorruptible.
Just as a becoming of motion would involve, as we saw, the existence of a process of change previous to the first, in the same way, a corruption of motion would involve the existence of a process of change subsequent to the last.
For when a thing ceases to move, it does not therefore at the same time cease to be movable.
For instance, the cessation of being burned does not involve the cessation of the capacity of being burned, since a thing may be capable of being burned without being burned.
Nor, when a thing ceases to be mover, does it therefore at the same time cease to be motive.
Again, the destructive agent will have to be destroyed after what it destroys has been destroyed, and then that which has the capacity of destroying it will have to be destroyed afterward, for being destroyed also is a kind of change.
If, then, the view which we are criticizing involves these impossible consequences, it is clear that motion is eternal.
971. After raising the problem of the eternity of motion, the Philosopher now intends to show that motion is eternal.
His treatment is divided into two parts: in the first, he explains his proposition; in the second, he solves objections contrary to his proposition, at “the arguments that may be” (252b7; [997]).
About the first, he does two things: first, he presents arguments to show the eternity of motion; second, he answers opinions to the contrary, at “it is clear then that motion” (252a4; [991]).
About the first, he does two things: first, he shows that motion always has been; second, that it always will be, at “the same reasoning” (251b28; [985]).
About the first, he does two things: first, he explains his proposition with an argument from motion; second, he uses an argument from time, at “further, how can there be” (251b10; [979]).
About the first, he does three things: first, he sets down something needed for his proposition; second, he presents a proof that manifests his proposition, at “moreover, these things also” (251a16; [976]); third, he shows that his argument proceeds necessarily, at “for some things cause” (251a28; [977]).
972. He first says, therefore (251a8), that, in order to demonstrate the proposition, we must begin with things determined at the very beginning of the Physics and use them as principles. By this, he gives us to understand that the preceding books, in which he determined about motion in general and that, for this reason, are universally called About Natural Things, are set off from this book 8, in which he begins to apply motion to things.
He assumes, therefore, what was said in Physics 3, namely, that motion is the act of a mobile precisely as such. From this, it appears that, in order for motion to exist, there must exist things that can be moved with some sort of motion, because an act cannot exist without the thing of which it is the act. Accordingly, from the definition of motion, it is evident that there must be a subject of motion if there is to be motion at all.
But, even without the definition of motion, that fact is per se evident from the general consent of all, for everyone admits as a necessary fact that nothing is moved except what can be moved, and this with reference to any and all motion. For example, nothing can be altered except what is alterable, or be moved with respect to place unless it be changeable with respect to place.
And, because the subject is by nature prior to what is in the subject, we can conclude that, in individual changes—both from the viewpoint of the mobile and from that of the mover—the combustible subject is prior to its being set afire, and the combustive, that is, the subject capable of setting it afire, is prior to its setting afire—prior, I say, not always in time, but in nature.
973. From this argument of Aristotle, Averroes took occasion to speak against what is held by faith about creation. For if coming to be is a kind of change and every change requires a subject, as Aristotle here proves, it is necessary that whatever comes to be does so from a subject; therefore, it is not possible for something to come to be from nothing.
He confirms this with another argument: when it is said that the black comes to be from the white, this is not to speak per se, in the sense that the white itself is converted into the black, but it is to speak per accidens, in the sense that, upon the departure of the white, the black succeeds it.
Now, whatever is per accidens is reduced to what is per se. But that from which something comes to be per se is the subject, which enters into the substance of what comes to be. Therefore, whatever is said to come to be from its opposite comes to be from it per accidens, but per se it comes to be from the subject. Accordingly, it is not possible for being to come to be from non-being absolutely.
In further support of his position, Averroes adduces the common opinion of the early philosophers that nothing comes to be from nothing.
He also gives two reasons from which he considers that the position arose that something should come to be from nothing.
The first is that ordinary people do not consider anything to exist but what is comprehensible by sight; therefore, because they see something visible come to be that previously was not visible, they think that it is possible for something to come to be from nothing.
The second reason is that, among the common people, it could be thought to be a weakening of the virtue of the agent that it should need matter in order to act. But that condition, however, does not derive from the impotency of the agent, but from the very account of motion. Therefore, because the first agent does not have a power that is in any way deficient, it follows that it should act without a subject.