Commentary Aristotle Physics
Lecture 4
Refutation of arguments seeming to prove motion is not eternal
The arguments that may be advanced against this position are not difficult to dispose of.
The chief considerations that might be thought to indicate that motion may exist but at one time it had not existed at all are the following.
First, it may be said that no process of change is eternal. For the nature of all change is such that it proceeds from something to something, such that every process of change must be bounded by the contraries that mark its course, and no motion can go on to infinity.
Second, we see that a thing that neither is in motion nor contains any motion within itself can be set in motion.
For instance, inanimate things that are not in motion but at rest (whether the whole or some part is in question) are at some moment set in motion;
but if motion cannot have a becoming before which it had no being, these things ought to be either always in motion or never.
Third, the fact that motion is not eternal is evident above all in the case of animate beings.
For sometimes there is no motion in us and we are quite still and nevertheless we are then at some moment set in motion—that is to say, sometimes we produce a beginning of motion in ourselves spontaneously without anything having set us in motion from without.
We see nothing like this in the case of inanimate things, which are always set in motion by something else from without.
The animal, on the other hand, we say, moves itself; therefore, if an animal is ever in a state of absolute rest, we have a motionless thing in which motion can be produced from the thing itself, and not from without.
Now, if this can occur in an animal, why should not the same be true also of the universe as a whole?
If it can occur in a small world it could also occur in a great one, and if it can occur in the world, it could also occur in the infinite; that is, if the infinite could as a whole possibly be in motion or at rest.
Of these objections, then, the first mentioned—that motion to opposites is not always the same and numerically one—is a correct statement.
In fact, this may be said to be a necessary conclusion, provided that it is possible for the motion of that which is one and the same to be not always one and the same.
(I mean that, for instance, we may question whether the note given by a single string is one and the same or is different each time the string is struck, although the string is in the same condition and is moved in the same way.)
But still, however this may be, there is nothing to prevent there being a motion that is the same in virtue of being continuous and eternal; we shall have something to say later that will make this point clearer.
As regards the second objection, no absurdity is involved in the fact that something not in motion may be set in motion, that which caused the motion from without being at one time present and at another absent.
Nevertheless, how this can be so remains matter for inquiry—how it comes about, I mean, that the same motive force at one time causes a thing to be in motion and at another does not do so. For the difficulty raised by our objector really amounts to this: why is it that some things are not always at rest, and the rest always in motion?
The third objection may be thought to present more difficulty than the others, namely, the objection that alleges that motion arises in things in which it did not exist before, adducing as proof the case of animate things: an animal is first at rest and afterwards walks, not having been set in motion perhaps by its surroundings.
This, however, is false, for we observe that there is always some part of the animal’s organism in motion, and the cause of the motion of this part is not the animal itself, but possibly its environment.
Moreover, we say that the animal itself originates not all of its motions, but its locomotion. So, it may well be the case—or rather we may perhaps say that it must necessarily be the case—that many motions are produced in the body by its environment and some of these set in motion the intellect or the sensible appetite, and that this again then sets the whole animal in motion.
This is what happens when animals are asleep: though there is then no perceptive motion in them, there is some motion that causes them to wake up again. But we will leave this point also to be elucidated at a later stage in our discussion.
997. After giving arguments proving that motion always exists, the Philosopher now intends to answer objections to the contrary.
About this, he does two things: first, he gives the arguments; second, he answers them, at of these objections (252b28; [1000]).
In regard to the first, he gives three arguments, after first stating that it is not difficult to solve the objections contrary to his position. And he says that there are three main arguments from which it seems to follow that motion began to be at some time after previously not existing at all.
The first of these is that whereby he proved in book 6 that no change is infinite, because by the same argument, it can be proved that no change is eternal. For no terminated change is eternal any more than it is infinite. But every change is terminated. For every motion is naturally from something to something, and these two are contraries; hence, of necessity, the termini of any change are the contraries within the sphere of that change. But, because contrariety of termini is not evident in all cases of local motion, Aristotle adds something common to every motion, namely, that nothing is moved to infinity, because nothing is moved to what it cannot reach, as has been said in book 6. Accordingly, it is clear that no motion is perpetual, just as it is not infinite. If, therefore, no motion is perpetual, it also seems possible to posit a time in which there is no change. This first argument is taken from motion.
998. The second argument is based on the mobile, at “second, we see” (252b12). If motion cannot newly come to be when previously it was not, it seems fitting to say of anything that it is either always in motion or never in motion. For if motion can sometimes be and sometimes not be in one particular mobile, why not for the whole universe? But we see that it is possible for something to be moved that previously was not moved as a whole and that had no motion in itself with respect to any of its parts, as is apparent in non-living things, among which some mobile begins at one time to be moved when previously no part nor the whole had been moved, but it was completely at rest. Thus, it remains that, in the whole universe, there can be motion where previously there was none.
999. But, because in non-living things, even though motion is seen to begin anew in something when previously there was none at all, yet motion appears to have been preexisting in something external by which it is moved, he accordingly gives a third argument from animals, which are moved not from without but by themselves. This argument is at “third, the fact” (252b17).
And he says that it is more evident in animals than in the non-living that motion begins after previously having not existed.
For when we have rested for a time such that no motion exists in us, we begin at a certain time to be moved, and the principle of our motion is from our very selves even if nothing external moves us.
This, however, does not happen in non-living things, because they are moved always by something external, such as the cause that generates them, or a cause that removes an obstacle, or a cause that subjects them to force.
It follows from this that, if an animal is at one time entirely at rest, then motion begins to exist in an immobile being after previously not existing in it and the motion does not originate from an external mover, but from the very thing that is moved. And, if this can occur in an animal, there is nothing to prevent its occurring in the universe. For an animal, and especially man, possesses a likeness to the world; therefore, it is said that man is a small world. Accordingly, if, in this small world, motion can begin after previously not existing in it, it seems that the same can happen in the large world. And, if this happens in the world, it can happen in the infinite whole, which some assumed exists beyond the world—provided, of course, that there is something infinite that can rest and be moved.
1000. Then, at “of these objections” (252b28), he answers these arguments in order. To the first, he says that it is correct to say that motion that occurs between contraries cannot always endure as one and the same numerical motion—for, as will be proven later, this may even be necessary. He leaves this in doubt because it has not yet been proved. But, because someone could say that even motion that is between contraries can be always numerically the same on account of maintaining the same mobile that is repeatedly moved from one contrary to another—for example, if it is first moved from white to black, and then from black to white, and so on throughout time—he then adds that it is not possible that a motion that is always in one and the same mobile be kept one and the same by repetition. And he explains this by an example. Let the same chord be continually struck on a lyre, and let the striker be unvarying in his striking; one may ask whether the motion and sound of the one chord struck twice is one and the same or continually different. Yet whatever be the case with other mobiles, there is nothing to prevent a motion that is not between contraries, such as a circular motion, from being the same continual and perpetual motion. This will be made clearer from later development. Therefore, although every motion is finite with respect to its termini, yet some motion can be continuous and perpetual by repetition.
1001. Then, at “as regards the second” (253a2), he answers the second argument. And he says that there is nothing unsuitable in a non-living thing beginning to be moved when previously it was not being moved, provided that this occurs because an external mover is present at one time and not at another. For it is clear that motion must preexist on the part of a mover that at some time becomes near but previously was not so.
However, this seems to be a point to be looked into as a problem, namely, whether, if a mover exists, the same object can be at one time moved by this mover and at another not. For he had previously said that such a thing cannot happen unless there intervenes some change affecting either the mover or the mobile. Accordingly, motion always preexists, whether or not a mover preexists. Now, this point seems to need investigation, because whoever proposed this argument seems to be certain about everything but one factor, namely, why it is that things at rest do not always rest and mobiles are not always in motion.
1002. Then, at “the third objection” (253a7), he answers the third argument. And he says that the third objection causes the greatest problem about whether motion can exist after previously not existing, based on what seems to happen in living things. For it seems that an animal that previously was at rest later begins to move about without any external cause of motion. Accordingly, it seems that that motion of the animal was not preceded by any motion, either in the animal or in anything else, as happens in non-living things.
But it is false that the motion of the animal does not come to be from something external. For we always observe in animals something naturally moved, not moved through will. And the cause of its being moved naturally is not the animal through its appetite, but perhaps the cause of this natural change is its surroundings, that is, the air, and beyond that the heavens, as is plainly the case when the body of an animal is altered by heat or coldness of air.
And he says perhaps because, in an animal, something is also moved naturally by an internal principle, as is evident in those changes that occur in the vegetal soul, such as are the digestion of food and the subsequent changes, which are called “natural” because they do not follow upon apprehension and appetite. And, because this seems to be contrary to what is proper to an animal, which is to move itself, he adds that, when we say that an animal moves itself, we do not understand this of any and every motion, but of local motion, according to which an animal moves itself through apprehension and appetite.
Accordingly, there is nothing to prevent many changes from taking place in the body of an animal on account of its surroundings, that is, the air and the heavens, some of which changes move the understanding or the appetite, by which in turn the whole animal is moved. Indeed, this is necessary.
1003. It should be noted that Aristotle here declares the way in which heavenly bodies act upon us. For they do not act directly on our souls, but on our bodies. But when our bodies are moved, then a change occurs per accidens in the powers of the soul, which are acts of bodily organs but not necessarily in the intellect and the intellective appetite, which do not use bodily organs. Yet the intellect and will sometimes follow upon some of these changes, as when a person, through his reason, chooses either to pursue or to reject or to do something on account of a passion that began in the body or in the sensitive part. Therefore, Aristotle does not say that all motions caused by the surroundings move the intellect or appetite, but that some of them do. In this way, he excludes necessity from the intellective powers.
Of the things he said, he gives an example from sleeping things, in which there seems to be maximum rest with respect to animal motions. But, even though there be no motion during sleep that is sensible—that is, proceeding from sense apprehension—animals rise awakened by some motion existing within, due either to the work of the nutritive soul (as when, as a result of the food’s being digested, the vapors that caused sleep vanish and the animal is aroused), or when the body is altered by its surroundings, from heat or cold.
Thus, it is clear to anyone who considers the matter diligently that no motion ever newly appears in us unless some other motion preceded. And he promises to give a fuller explanation of this later.