Commentary Aristotle Physics
978. Then, at “but, at any rate” (251b1), he sets forth the proof that manifests his proposition. Therefore, he says that, from the fact that things are such—that is, that a similar situation prevails with respect to things that act by nature and things that act by intellect—then, speaking universally of all, we can say that whatever things are possible to make, or to be acted upon, or to cause motion, or to be moved, are capable of it not under all conditions; that is, they cannot cause motion or be moved in just any disposition in which they find themselves, but according as they are in some definite state and nearness with respect to each other.
And this he concludes from the premises, because it has already been said that, both in things that act according to nature and in things that act according to will, nothing is the cause of diverse things except as it is a different state.
Accordingly, it is necessary that, when the mover and the moved approach one another according to a suitable distance and, likewise, when they are in whatever disposition is required for one to cause motion and for the other to be moved, then the one must be moved and the other must cause motion.
If, therefore, there was not always motion, it is clear that existing things were not in that state that allowed for one to cause motion and another to be moved; rather, they were in the state of not being able to cause motion and of being moved at that time. But they later reached that state in which one moves and the other is moved. Therefore, one or the other of them changed.
For we see that, in all things that are said to be relative, it does not happen that a new relation arises except through a change affecting one or the other or both—as, for example, if something that previously was not double has now become double, even though not both of the extremes were changed, yet at least one of them was. Accordingly, if there newly arises a relationship by which something causes motion and something is moved, then one or the other or both had to be previously moved. Hence, it follows that there is a change prior to the one assumed to be the first.
979. Then, at “further, how can there be” (251b10), he explains his proposition with an argument from time.
First, he sets down two things necessary for his proposition.
The first of these is that prior and subsequent cannot occur unless there is time, since time is nothing else than prior and subsequent precisely as numbered.
The second is that, time cannot exist unless there is motion. This too is clear from the definition given in book 4, describing time as the number of motion with respect to prior and subsequent.
980. Second, at “if, then, time” (251b12), he argues for a conditional proposition from statements made in book 4. For there, according to his doctrine, he stated time to be the number of motion; according to the doctrine of the other philosophers, time is a motion, as he there stated. But, whichever of these is true, it follows that this conditional is true: if time always exists, it is necessary that motion be perpetual.
981. Third, at “but, so far as time” (251b13), he proves in two ways the antecedent of this conditional.
He proves it first from the opinions of others. And he says that all the philosophers but one, Plato, seem to be in accord with regard to the opinion that time is not begotten, that is, that it did not begin to exist after previously not existing. From this, Democritus also proved that it is impossible that all things should have been made in the sense of newly beginning to be, because it is impossible that time have been so made that it begin newly to be.
Only Plato generates time, that is, says that time was newly made. For he says that time was made at the same time as the heavens, and he supposed that the heavens had a generation, that is, that they have a beginning of their duration, as Aristotle here claims, and as Plato’s words seem at first glance to indicate—although Platonists say that Plato asserted that the heavens were made in the sense that they have an active principle of their existence but not as having a principle of their duration.
Thus, therefore, does Plato alone seem to have conceived that time cannot be without motion, for he did not suppose that time existed before the motion of the heavens.
982. Second, at “now, since time” (251b17), he proves the same point by an argument from the fact that it is impossible to say or to understand that time exists without the now, just as it is impossible that there be a line without a point. The now, however, is something intermediate, having as part of its account that it be at once a beginning and an end—that is, the beginning of a future time but the end of a past. From this, it appears necessary for time always to be. For whatever time is taken, its boundary is a now in both senses. And this is clear from the fact that nothing is actual in time but the now, because what is past has gone by and what is future does not yet exist. But the now that is taken as the boundary of time is both a beginning and an end, as has been said. Therefore, it is necessary that, from both aspects of whatever time is taken, time always be; otherwise, the first now would not be an end and the last not a beginning.
But, from the fact that time is eternal, he concludes that motion too must be eternal; the reason for this conclusion being that time is a property of motion, for it is its number, as was said.
983. But the argument of Aristotle does not appear efficacious. For the now is to time as the point is to the line, as was explained in book 6. But it is not necessary that a point be an intermediate, for some points are merely the beginnings of lines and others the ends, although every point would be both a beginning and an end if the line were infinite. One could not, therefore, prove that a line is infinite from the fact that every point is a beginning and an end.
Rather, it is the other way around: from the fact of a line’s being infinite, one would go on to prove that every point would be both a beginning and an end.
Accordingly, it also appears that the claim that every now is a beginning and an end is not true unless time is assumed to be eternal. Therefore, in assuming this as a middle term (namely, that every now is a beginning and an end), Aristotle seems to suppose the eternity of time—the very thing he ought to prove.
Now, Averroes, in trying to save Aristotle’s argument, says that the attribute of always being both a beginning and an end belongs to the now inasmuch as time is not stationary like a line, but flowing. But this does not pertain to the proposition. For from the fact that time is flowing and not stationary, it follows that one now cannot be taken twice in the way that one point is taken twice, but the flow of time has nothing to do with the now being at once a beginning and an end. For the account of beginning and end is the same in all continua, whether they be permanent or flowing, as is clear from book 6.
984. And, therefore, another explanation must be furnished in accord with the intention of Aristotle, which is that he wishes to derive the fact that every now is a beginning and an end from what he had first supposed, namely, that prior and subsequent would not exist if time did not exist. For he uses this principle, which he supposes for no other purpose, but deduces from it that every now is a beginning and an end.
For let us suppose that some now is the beginning of a time; but it is clear from the definition of a beginning that the beginning of a time is that before which nothing of the time existed. Therefore, there must be taken something before or prior to the now that is assumed as the beginning of the time. Prior, however, does not exist without time. Therefore, the now that is taken as the beginning of a time is also the end of a time. In the same way, if the now is given as the end of a time, it follows that it will also be a beginning, because an end is by definition that after which nothing of a thing exists; but after cannot be without time. Therefore, it follows that the now that is the end of a time is also a beginning.
985. Then, at “the same reasoning” (251b28), he shows that motion will always be. And he shows this on the part of motion, because the argument from motion given above concluded only that motion never began, whereas the argument from time concluded both that it never began and that it never ceases. He says, therefore, that the very argument by which it was proved that motion never began can prove that motion is incorruptible, that is, that it will never end. For just as it followed from the assumption that motion began that there was a change prior to the change assumed to be first, so too, if it be supposed that motion at some time ceases, it follows that a change will occur after the one assumed to be the last.
How this follows he explains by abbreviating the more diffuse explanation he gave with regard to the beginning of motion. For he had supposed that, if motion began, the mobiles and movers either began or always were. The same alternatives can be taken here: if motion should cease, the mobiles and movers will remain or they will not. But, because he had previously shown that the same conclusion follows from either alternative, therefore he uses only the one alternative here, namely, the supposition that motion ceases in such a way that the mobiles and movers also pass away.
Therefore, beginning with the assumption mentioned, he says that both the actual motion and the mobile do not cease to move—that is, pass away—simultaneously; but, just as the generation of a mobile is prior to its motion, so the ceasing to be of a mobile is subsequent to the passing away of its motion. This is so because something combustible can remain after combustion ceases.
And what was said of the mobile must also be said of the mover, because a mover in act does not, in ceasing to be, cease at the same time to be a mover in potency. Accordingly, it is evident that, if even the mobile ceases to be after the destruction of its motion, then there has to be a process by which the mobile passes out of existence.