Commentary Aristotle Physics
Lecture 15. Only local motion can be continuous and eternal
We have now to show which kind of locomotion is primary. The same method will also make clear at the same time the truth of the assumption we have made both now and at a previous stage, that it is possible that there should be a motion that is continuous and eternal.
Now, it is clear from the following considerations that no other motion than locomotion can be continuous.
Every other motion and change is from an opposite to an opposite. For the processes of generation and corruption, the limits are the existent and the non-existent; for alteration, the limits are the various pairs of contrary passions; and for increase and decrease, they are either greatness and smallness or perfection and imperfection of magnitude. Changes to the respective contraries are contrary changes.
Now, a thing that is undergoing any particular kind of motion that it has not always undergone, though it has previously existed, must previously have been at rest so far as that motion is concerned. Thus, it is clear that, for the changing thing, the contraries will be states of rest.
And we have a similar result in the case of changes that are not motions, for generation and corruption, whether regarded simply as such without qualification or as affecting something in particular, are opposites.
Therefore, provided it is impossible for a thing to undergo opposite changes at the same time, the change will not be continuous, but a period of time will intervene between the opposite processes.
The question whether these contradictory changes are contraries or not makes no difference, provided only it is impossible for them both to be present to the same thing at the same time; the point is of no importance to the argument.
Nor does it matter whether the thing need not rest in the contradictory state, or whether there is no state of rest as a contrary to the process of change. It may be true that the non-existent is not at rest and that corruption is a process to non-existence.
All that matters is the intervention of a time: it is this that prevents the change from being continuous. So, too, in our previous instances, the important thing was not the relation of contrariety, but the impossibility of the two processes being present to a thing at the same time.
And there is no need to be disturbed by the fact that, on this showing, there may be more than one contrary to the same thing or that a particular motion will be contrary both to rest and to motion in the contrary direction.
We have only to grasp the fact that a particular motion is in a sense the opposite both of a state of rest and of the contrary motion, in the same way as what is of equal or standard measure is the opposite both of what surpasses it and of what it surpasses, and that it is impossible for the opposite motions or changes to be present to a thing at the same time.
Furthermore, in the case of generation and corruption, it would seem to be an utterly absurd thing if, as soon as anything has become, it must necessarily perish and cannot continue to exist for any time.
And, if this is true of generation and corruption, we have fair grounds for inferring the same to be true of the other kinds of change, since it would be in the natural order of things that they should be uniform in this respect.
1097. After proving that local motion is the first of all motions, the Philosopher now shows which local motion is the first.
And because, as he said above, the motion must be the same that is continuous and first, this treatment is divided into two parts:
first, he shows which motion can be always continuous;
second, he shows that such a motion is the first, at it can now be shown (265a13; [1134]).
The first part is divided into three sections:
in the first, he shows that no motion but local can be continuous;
in the second, he shows that no local motion but a circular one can be continuous, at let us now proceed (261b27; [1104]);
in the third, he shows that a circular motion can be continuous, at on the other hand (264b9; [1129]).
About the first, he does two things:
first, he proposes what he intends;
second, he proves his proposition, at every other motion (261a32; [1098]).
He thus says first that, since it has been shown that change of place is the first among all types of motion, we must now show which change of place is first, because there are many types of local motion, as was proved in book 7.
And at the same time, according to the same method, namely, art—that is, according to the same technical consideration—it will be plain what we have just said and what was also previously assumed at the beginning of book 8: there exists a motion that is continuous and perpetual. Now, the first and the continuous must be the same, as was proved above. For that reason, they fall under the same consideration. But that no type of motion other than local motion can be continuous and perpetual will be clear from what will be said.
1098. Then, at “every other motion” (261a32), he proves the proposition.
And about this, he does two things:
first, he shows that no other species of change but local can be continuous and perpetual, remaining one and the same;
second, he shows that two changes that are opposite cannot succeed one another without an interval of rest, at furthermore, in the case (261b22; [1103]).
About the first, he does two things:
first, he proves the proposition;
second, he excludes some objections, at the question whether (261b7; [1100]).
About the first, he does two things:
first, he proves the proposition in motions;
second, he proves the proposition in changes, at “and we have a similar” (261b3; [1099]).
He therefore first proposes (261a32) one proposition that is true both for changes and motions, namely, that all changes and motions are from opposites to opposites. But local motion is, in a sense, excluded from this generality, as was said at the end of book 6. Generation and ceasing to be, which are changes, have existent and non-existent for their termini; the opposite termini of alteration are contrary passions, that is, passible qualities, such as hot and cold and black and white; and the opposite termini of growth and decrease are large and small or perfect and imperfect in magnitude or quantity.
But it is plain from what was said in book 5 that motions toward contrary termini are contrary. Therefore, a motion to white is contrary to a motion to black. But contraries cannot be together; therefore, while something is being moved to white, it cannot at the same time be undergoing a motion to black.
Hence, what begins to be moved from white to black by the motion of blackening, even if it should be moved by the motion of whitening while becoming white, could not simultaneously be moved by the motion of blackening. But what was existing previously, if it was not always being moved by some definite motion, must be considered as having been previously resting with a rest opposed to this motion, for whatever is apt to be moved is either at rest or being moved.
Therefore, it is plain that what is being moved to a contrary was at one time resting with a rest opposed to that motion. Hence, no motion to a contrary can be continuous and perpetual.
If, therefore, to this conclusion be added what was first assumed—namely, that every motion of alteration, growth, or decrease is to a contrary—it follows that none of these motions can be continuous and perpetual.
1099. Then, at “and we have a similar” (261b3), he proves the same thing of changes, that is, for generation and ceasing to be. These, indeed, are opposed both universally according to the common opposition of being and non-being, and also in the singular thing, as the generation of fire is opposed to the ceasing to be of fire according to the opposition of its existence and its non-existence.
Hence, if opposite changes cannot coexist, it will follow that no change is continuous and perpetual in the same way that it followed previously for motions, and that, between two generations of the same thing, there must intervene a time in which ceasing to be occurred. In like manner, a time of generation interrupts instances of ceasing to be.
1100. Then, at “the question whether” (261b7), he dismisses three objections.
First of all, someone could say that, since changes are opposed according to the opposition of their termini, whereas the termini of generation and ceasing to be are not contrary, but contradictory, it seems to follow that generation and ceasing to be are not contrary; consequently, the same argument will not apply to them and to motions that are contrary.
To this objection, he replies that it makes no difference whether changes that differ according to contradictory termini are contrary or not contrary, as long as this alone is true: that it is impossible for both to be in the same thing at the same time. For to be contrary or not contrary has no bearing on the argument given.
1101. The second objection he dismisses at “nor does it matter” (261b10). Someone could say that it is necessary for what is not always being moved to be previously at rest, because motion is the opposite of rest. But this does not occur in generation and ceasing to be, to which, properly speaking, rest is not opposed, as was said in book 5.
To this objection, he responds that it makes no difference to the argument given whether there is rest in either of the contradictory termini or not, or whether change is not contrary to rest (because perhaps what does not exist cannot rest, and ceasing to be tends to non-existence, from which it seems that rest cannot occur in the terminus of a ceasing to be). But the proposition is sufficiently proved if an intermediate time exists between two generations or two instances of ceasing to be. For the consequence will be that neither of these changes is continuous.
After this, he returns once more to the first objection and says that the reason why it makes no difference whether the changes between contradictory termini are contrary or not is that, in the earlier discussions about motions, it was not the question of contrariety that played a part in the proofs, but the fact that the two changes could not occur at one and the same time. And this is not a peculiarity of contraries, but is common to all opposites.
1102. The third objection he dismisses at “and there is no need” (261b15). For he had said previously that motions that tend to contraries are contrary. Therefore, since motion is contrary to rest, it seems to follow that one thing has two contraries—which is impossible, as is proved in Metaphysics 10.
In order to exclude this, he says that there is no need to be disturbed about the fact that one thing seems to be contrary to two things—that is, a motion contrary to rest and to the motion that is to a contrary.
Rather, the only thing we ought to take is that one contrary motion is in some manner opposed both to another contrary motion and to rest: to another contrary motion according to direct contrariety; but to rest more according to privative opposition. Yet this latter opposition has some contrariety, inasmuch as an opposite rest is the end and complement of a contrary motion, just as “equal and commensurable” is opposed in a way to two things, to the excelling and to what is excelled—that is, to the large and the small—to which two it is opposed more according to privation, as is plain in Metaphysics 10. And once more, what is important to grasp is that opposite motions or opposite changes do not occur at one and the same time.
1103. Then, at “furthermore, in the case” (261b22), he shows not only that there must be a time between two motions or changes of the same species, and that no single change that tends to one of two opposites can be perpetual and continual, but also that it is impossible for opposite motions or changes so to follow one upon the other such that there is no time between them. For it seems to be utterly at odds with generation and ceasing to be that, when something has come to be and its generation is complete, it necessarily begins immediately to cease to be, such that there would be no period of time in which the generated thing would be permanent. For a thing would be generated in vain if the generated thing were not to remain in existence.
Hence, from these changes of generation and ceasing to be, we can understand the others. For the natural is what occurs in a like way in all things, since nature always acts in the same way. Therefore, just as it seems unacceptable for something to cease to be as soon as it is generated, so too it seems unacceptable that a thing should start becoming black as soon as it became white, or that a thing should begin to shrink as soon as it is grown. For in all these cases, the intention of nature would be frustrated.