Commentary Aristotle Physics
But the same now is in two periods of time, in one of which the mobile is totally at rest and in the other of which it is totally in motion (as appears in that which is in motion after rest or at rest after motion). Therefore, if something is apt to rest and be in motion in the now, it will follow that something is at once in motion and at rest, which is impossible.
The third argument is given at “again, when we say” (234b5). Rest is said of things that maintain themselves now just as they were previously, both in their entirety and in respect of all their parts. For it is on this account that a thing is said to be in motion: that now it is different from what it was previously in respect to place or quantity or quality. But, in the now itself, there is nothing previous; otherwise, the now would be divisible, because the word “previous” refers to the past. Therefore, it is impossible to rest in the now.
From this, he further concludes that necessarily anything that is being moved and anything that is at rest is being moved and is at rest in time.
796. Then, at “further, what is being changed” (234b10), he shows that, whatever is in motion is divisible. For every change is from this to that. But, when something is at the goal, it is no longer being changed, but has been changed, for nothing can be at the same time in the state of being changed and in that of having been changed, as was said above. But, when something is at the starting point of change both in its entirety and in regard to all its parts, then it is not being changed; for it was said above that whatever maintains itself constant in its entirety and in regard to all its parts is not being changed, but is at rest.
He adds, and all its parts, because, when a thing is beginning to be changed, it does not emerge in its entirety from the place it previously occupied, but part emerges after part.
Moreover, it cannot be said that it is in both terms in its entirety and in regard to its parts while it is being moved; for then something would be in two places at one time.
Nor, again, can it be said that it is in neither of the termini, for we are now speaking of the nearest end into which a thing is being changed and not of the remotest; for example, if something is being changed from white to black, black is the remote end, but grey is the nearer one. In like manner, if a line ABCD is divided into three equal parts, it is clear that a mobile that, in the beginning of the motion, was in AB as in a place equal to itself can be neither in AB nor in CD during the motion; for at some time, it is in its entirety in BC.
Therefore, when it is said that what is being moved cannot happen to be in either extremity while it is being moved, this must be understood as referring not to the remotest extremity, but to a nearer one.
What is left, therefore, is that whatever is being changed is partly in one and partly in the other while it is being changed; for example, when something is being changed from AB to BC, during the motion, the part leaving the place AB is entering the place BC; likewise, when something is being moved from white to black, the part that ceases to be white becomes grey or light grey.
Consequently, it is clear that anything that is being moved, since it is partly in one and partly in the other, is divisible.
797. But it should be mentioned that the Commentator here raises the problem that, if Aristotle does not intend in this place to demonstrate that every mobile is divisible, but only what is mobile in regard to motion (which he said is present in only three genera: quantity, quality and place), then his demonstration will not be universal but particular, because even the subject of substantial change is found to be divisible. Hence, he seems to be speaking of what is subject to any and every type of change, including even generation and ceasing to be in the genus of substance. And this is evident from his very words: he does not say what is being moved, but what is being changed.
But, in that case, his demonstration has no value, because some changes are indivisible, such as generation and ceasing to be of substance, which do not consume time. In such changes, it is not true that what is being changed is partly in one extremity and partly in the other, for when fire is generated, it is not partly fire and partly non-fire.
798. In the face of this problem, he proposes a number of solutions, one of which is Alexander’s, who says that no change is indivisible or in non-time. But this must be rejected because it conflicts with an opinion that is held as probable and famous with Aristotle and all Peripatetics: certain changes are in non-time, such as illumination and the like.
The Commentator mentions also the solution of Themistius, who says that, even if there be changes in non-time, they are hidden, whereas Aristotle appeals to what is evident, namely, that change occurs in time. But this he also rejects, because change and the changeable are divided in the same way, and the divisibility of a mobile is more hidden than the divisibility of change. Hence Aristotle’s demonstration would not be valid, because someone could say that, although things that change by changes evidently divisible are themselves divisible, yet there are some hidden changeable beings that are indivisible.
He mentions, too, the solution of Avempace, who says that the problem here is not about the quantitative division of the things capable of change, but of that division whereby the subject is divided by contrary accidents, one of which is changed into the other.
799. Then the Commentator adds his own solution: those changes that are said to occur in non-time are the extremities of certain divisible motions. It happens, therefore, that something should be changed in non-time, insofar as every motion is terminated in an instant. And, because what is accidental is ignored when it comes to demonstrating, Aristotle proceeds in this demonstration as though every change were divisible and in time.
800. But, if you consider the matter correctly, you will see that this objection is not to the point. For in his demonstration, Aristotle does not use as his principle the statement that every change is divisible (since he proceeds rather from the divisibility of the mobile to the division of change, as will be clear later; for as he says later, divisibility is first in the mobile before it is in motion or change).
Rather, he uses principles that are evident and must be conceded in any and every case of change: what is being changed in regard to a certain matter is not being changed in regard to that matter as long as it is totally and according to all its parts still in the starting point; and, when it is in the goal, it is not being changed, but has been changed; and it cannot be entirely in both terms or entirely in either of them, as was explained. Hence, it necessarily follows that, in any change whatsoever, what is being changed is, during the change, partly in one extremity and partly in the other.
But this occurs in various ways in various changes. For in changes between whose extremities there is something intermediate, it can happen that the mobile is, during the change, partly in one extreme and partly in the other extreme precisely as extremes. But, in those between whose extremes there is nothing intermediate, that which is being changed does not have different parts in different extremities precisely as extremities, but by reason of something connected with the extremities.
For example, when matter is being changed from privation of fire to the form of fire, then while it is in the state of being changed, it is indeed under privation as to itself, but it is yet partly under the form of fire, not inasmuch as it is fire, but according to something connected with it—that is, according to the particular disposition for fire, which disposition it partly receives before it has the form of fire. That is why Aristotle will later prove that even generation and ceasing-to-be are divisible, because what is generated was previously being generated and what ceases to be was previously ceasing to be.
Perhaps this was the sense in which Alexander understood the statement that every change is divisible: either according to itself or according to a motion connected with it. So also Themistius understood by the statement that Aristotle took what was evident and abstracted from what was hidden that the proper place for treating of the divisibility or indivisibility of changes would not be reached until later.