Commentary Aristotle Physics
We must grasp the fact, therefore, that animals move themselves only with one kind of motion, and that this is not strictly originated by them.
The cause of it is not derived from the animal itself, it is connected with other natural motions in animals, which they do not experience through their own instrumentality, such as increase, decrease, and respiration.
These are experienced by every animal while it is at rest and not in motion in respect of the motion set up by its own agency; here the motion is caused by the atmosphere and by many things that enter into the animal.
Thus, in some cases, the cause is nourishment: when it is being digested, animals sleep, and when it is being distributed through the system, they awake and move themselves, the first principle of this motion being thus originally derived from outside.
Therefore, animals are not always in continuous motion by their own agency: it is something else that moves them, itself being in motion and changing as it comes into relation with each several thing that moves itself.
(Moreover, in all these self-moving things, the first mover and cause of their self-motion is itself moved by itself, though in an accidental sense—that is to say, the body changes its place, such that what is in the body changes its place also and is a self-mover through its exercise of leverage.)
Hence we may confidently conclude that, if a thing belongs to the class of unmoved movers that are also themselves moved accidentally, it is impossible that it should cause continuous motion.
So, the necessity that there should be motion continuously requires that there should be a first mover that is unmoved even accidentally, if, as we have said, there is to be in the world of things an unceasing and undying motion and the world is to remain permanently self-contained and within the same limits. For if the first principle is permanent, the universe must also be permanent, since it is continuous with the first principle.
We must distinguish, however, between accidental motion of a thing by itself and such motion by something else, the former being confined to perishable things, whereas the other case belongs also to certain first principles of heavenly bodies—of all those, that is to say, that experience more than one locomotion.
And, further, if there is always something of this nature—a mover that is itself unmoved and eternal—then that which is first moved by it must be eternal.
Indeed, this is clear also from the consideration that there would otherwise be no generation and corruption and no change of any kind in other things, which require something that is in motion to move them. For the motion imparted by the unmoved will always be imparted in the same way and be one and the same, since the unmoved does not itself change in relation to that which is moved by it.
But what is moved by something that, though it is in motion, is moved directly by the unmoved stands in varying relations to the things that it moves, such that the motion that it causes will not be always the same: by reason of the fact that it has contrary positions or forms at different times, it will produce contrary motions in each several thing that it moves and will cause it to be at one time at rest and at another time in motion.
The foregoing argument, then, has served to clear up the point about which we raised a difficulty at the outset—why is it that, instead of all things being either in motion or at rest, or some things being always in motion and the remainder always at rest, there are things that are sometimes in motion and sometimes not?
The cause of this is now plain: it is that, while some things are moved by an eternal unmoved mover and are therefore always in motion, other things are moved by a mover that is in motion and changing, such that they too must change.
But the unmoved mover, as has been said, since it remains permanently simple and unvarying and in the same state, will cause motion that is one and simple.
1077. After showing that the first mover is perpetual and utterly immobile on account of the perpetuity of the generation and corruption of animals, which move themselves, the Philosopher now intends to prove the same with an argument based on the moving principles.
About this, he does three things:
first, he reviews things said from the beginning of this treatise;
second, from these, he forms an argument for his proposition, at hence we may confidently (259b20; [1081]);
third, he finishes the solution of a doubt mentioned above, at the foregoing argument (260a11; [1085]).
1078. About the first, he reviews three things.
First (259a21), he reviews the destruction of certain improbable positions. And he says that anyone can know that there is a first immobile mover not only from the foregoing, but also by considering the principles of motion. And, as was said above, it is evident to sense that, among natural things, some are found that are at one time being moved and at another time at rest.
From this, it was explained above that none of these three positions is true:
the position that all things are always being moved;
the position that all things are always at rest;
and the position that all things that rest always rest, and that those being moved are always being moved.
The truth of this matter is demonstrated by the very things found under both—namely, under motion and under rest—since they have the potency to be moved at one time and to be at rest at another.
1079. Second, he recalls the process he went through when investigating the first immobile mover, at “the existence of things” (259a27). And he says that, because things that at one time are being moved and at another time are at rest are plain to all, lest anyone follow a fourth position—saying that all beings are such that they are at one time being moved and at another time at rest—we want to demonstrate two differing natures by showing that there are certain things that are always immobile and certain things, again, that are always being moved.
And, in dealing with this matter, we proposed first that whatever is being moved is being moved by something, and that this thing by which something is being moved is either immobile or is itself being moved, and if it is being moved, then it is moved either by itself or by another. And, since one cannot proceed to infinity in the series of being moved by another, we must come to this: there is some first principle of motion such that, in the genus of things that are moved, there is a first principle that moves itself, and beyond that, absolutely among all, there is a first principle that is immobile. Nor ought it to be thought strange that something move itself, because we plainly see many such in the genus of living things and animals.
1080. Third, at “this being so” (259b3), he recalls an objection mentioned and solved above. For since he had proved the eternity of motion, he cited to the contrary an objection based on living things that, after having been at rest, begin at a certain time to be moved. And what he says here is that those living things that move themselves seem to foster the opinion that, in the entire universe, motion begins after previously not having been, on the ground that we see this happen in things, namely, that they at one time begin to be moved, when previously they were not being moved.
To solve this, it is necessary to accept that animals move themselves with respect to one motion, namely, local motion, for only this motion, based on appetite, is found in animals. And yet animals do not properly move themselves even with respect to this motion as though another cause of this motion does not preexist. For no animal is of itself the first cause of being moved locally, but other motions precede—not voluntary but natural—either from within or from without, according to which the animals do not move themselves, as is plain in the motions of growth and decrease, and respiration, according to which animals are moved even though they rest with respect to local motion, by which they are moved by themselves.
The cause of these local motions is either an extrinsic container—namely, the heavens and air—by which the bodies of animals are changed externally, or is something that enters the bodies of animals, as air enters through breathing and as food enters through eating and drinking.
And, from such changes, caused either from within or from without, it happens that animals at a certain time begin to be moved, when previously they were not being moved, as is plain from the change that arises from food, for while the food is undergoing heat, the animals sleep on account of the vapors being broken down, but when the food is now digested and dissolved, and the vapors are left, the animals awaken and get up and move themselves from place to place. In all this, nevertheless, the first principle of motion is something extrinsic to the nature of the animal that moves itself.