Commentary Aristotle Physics
818. After explaining how motion is divided, the Philosopher now discusses the order of the parts of motion.
First, he asks whether there is a first in motion;
second, he shows how the factors involved in motion precede one another, at “now, everything that changes” (236b19; [826]).
About the first, he does two things:
first, he shows that that into which something is first changed is indivisible;
second, he shows how a first can and cannot be found in motion, at “but there are two” (236a7; [822]).
About the first, he does two things:
first, he mentions things necessary for explaining the proposition;
second, he proves the proposition, at the “primary when” (235b32; [821]).
About the first, he does two things:
first, he mentions his proposition;
second, he proves it, at “for what changes” (235b8; [819]).
819. He says first (235b6) that, because whatever is being changed is being changed from one terminus to the other, when the subject of change has now been changed, it has to be in the terminus to which.
Then, at “for what changes” (235b8), he proves this proposition with two arguments, the first of which is particular, and the second universal.
The first argument is this: either everything being changed must be distant from the terminus at which the change starts (as is evident in local motion, in which the place from which the motion starts remains and the mobile gets to be distant from it) or the terminus from which must cease to be, as in the motion called alteration, for when something white becomes black, the whiteness ceases to be.
In order to explain this proposition, he adds that either the process of being changed is the same as departing or the latter is a consequence of change and, therefore, having left (from the terminus from which) is a consequence of having been changed. But it is evident that they are the same in reality but different in account. For departing is spoken of in relation to the terminus from which, whereas change gets its name from the terminus to which. And in explanation of this, Aristotle adds that both are related to both in a similar way: as departing is related to being changed, so having departed is related to having been changed.
From these premises, he argues to the conclusion, using as his example the species of change that involves terms contradictorily opposed, where the transition is between being and non-being, as in generation and ceasing to be.
For it is evident from the foregoing that whatever is being changed departs from the terminus from which and that whatever has been changed has already departed. When, therefore, something has been changed from non-being to being, it has already departed from non-being. But, of anything at all, it is true to say that it either is or is not.
Therefore, what has been changed from non-being to being is in being when the change is over. Likewise, what has been changed from being to non-being must be in non-being. Therefore, it is evident that in the change that involves contradictories, the thing that has been changed exists in that into which it has been changed. And, if it is true in that type of change, then for an equal reason, it is true in other changes. From this, the first proposition is clear.
820. Then, at “moreover, if we take” (235b18), the second argument, a general one, is given. And he says that the same conclusion can be proved by considering any change at all; he makes this clear through local motion. Whatever has been changed must be somewhere, that is, either in the terminus from which or in some other. But, since what has been changed has already departed from that from which it has been changed, it must be elsewhere. Therefore, it must be either in that in which we are trying to prove it is—that is, in the terminus to which—or elsewhere. If it is in the former, our point is proved; if not, then let us suppose that something is being moved into B and, when the change is finished, the thing is not in B, but in C. Then we must say that, from C, it is also changed into B, because there is no interval between C and B. For a change of the type under discussion is continuous, and in continua, one part is not consecutive to another, for between two parts, there occurs a part that is similar to those two, as was proved above.
Hence, it will follow that, if what has been changed is in C when it has been changed and, from C, it is being changed to B (which is the terminus to which), then when it has been changed, it is also being changed into what it has already become—which is impossible. For being changed and having been changed are never simultaneous, as we have shown above. Now, it makes no difference whether the termini C and B are applied to local motion or to any other change. Consequently, it is universally true that what has been changed is (when it has been changed) in that into which it has been changed, that is, in the terminus to which.
From this, he further concludes that what has been made, when it has been made, has being, and that what is corrupted, when it is corrupted, is not being. For this has been shown universally of every change, and this is most manifest in changes that are according to contradiction, as is clear from what has been said.
Thus, it is manifest that what has been changed, as soon as it has been changed, is that into which it has been changed. He added as soon as because, after it has been changed into something, it could depart from it and not be there; but, as soon as it has been changed, it must be there.
821. Then, at “we will now show” (235b30), he shows that to have been changed is first and per se in an indivisible, and he says that that time in which what has been changed was first changed must be atomic, that is, indivisible. He explains why he adds first by saying that A is said to have been first changed as soon as it is not said to have been changed merely by reason of any of its parts. For example, if we say that a mobile has been changed in a day because it was changed in some part of the day, then it was not first changed in the day.
But that the time in which something has been first changed is indivisible he now proves. If the said time were divisible, let it be AC, and let it be divided at B. Now, three things are possible: either the change is over in each part of the time, or it is going on in each part, or in one part it is going on and in the other it is over.
Now, if it is over in each part, then it was first completely changed not in the whole, but in the part; but if it is being changed in each part, then it is also being changed in the whole (for the reason that something is said to be changing in a whole period of time is that the change was going on during each part of the whole time). But this is against our assumption that, in the whole of AC, it had been changed.
On the other hand, if it be supposed that, in one part of the time, it is being changed and, in the other part, it has been changed, the same difficulty ensues: it was not first changed in the whole time, for since the part is prior to the whole and something is in motion in a part of time before it is moved in the entire time, it follows that there was something prior to the first, which is impossible. Consequently, it must be admitted that the time in which the thing was first completely changed is indivisible.
From this, he further concludes that everything that has ceased to be and everything that has been completely made were made and ceased to be in an indivisible of time, because generation and ceasing to be are the termini of alteration. Consequently, if a motion is terminated in an instant (for these two things are the same—that is, the termination of a motion and to have been first changed), it follows that generation and ceasing to be occur in an instant.
822. Then, at “but there are two” (236a7), he shows how to find the first thing in a motion.
About this, he does two things:
first, he proposes the truth;
second, he proves it, at “for suppose that AD” (236a17; [823]).
He says first (236a7) that the expression in which something has been first changed has two interpretations:
first, it can mean that in which the change is first complete or terminated—in which case, it is true to say that something has been changed when the change is now over;
second, it can mean that in which it first began to be changed, and not that in which it was first true to say that it has been changed.
Taken in the first sense—according to the termination of the change—it is applied to instances of motion in which there exists a first in which something has been changed. For a change can be first terminated at some time, because every change has a termination. It was in this sense that we understood that that in which something was first changed is an indivisible, which was proved on the ground that it is the end—that is, the terminus, of the motion—and we know that every terminus of a continuum is an indivisible.