Commentary Aristotle Physics
Lecture 7. Whatever is moved is moved by another
Now, of things that cause motion or suffer motion, some are moved accidentally, others essentially.
Thus, motion is accidental to what merely belongs to or is a part of a thing that causes motion or suffers motion, but essential to a thing that causes motion or suffers motion not merely by belonging to such a thing or containing it as a part.
Of things to which the motion is essential, some derive their motion from themselves, but others from something else; and, in some cases, their motion is natural, while in others, violent and unnatural.
Thus, in things that derive their motion from themselves—for instance, all animals—the motion is natural (for when an animal is in motion, its motion is derived from itself). And, whenever the source of the motion of a thing is in the thing itself, we say that the motion of that thing is natural.
Therefore, the animal as a whole moves itself naturally; but the body of the animal may be in motion unnaturally as well as naturally, depending upon the kind of motion that it may happen to be suffering and the kind of element of which it is composed.
And the motion of things that derive their motion from something else is in some cases natural, but in other unnatural; for instance, upward motion of earthy things and downward motion of fire are unnatural.
Moreover, the parts of animals are often in motion in an unnatural way, their positions and the character of the motion being abnormal.
The fact that a thing that is in motion derives its motion from something is most evident in things that are in motion unnaturally, for in such cases it is clear that the motion is derived from something other than the thing itself.
Next to things that are in motion unnaturally, those whose motion, while natural, is derived from themselves—for instance, animals—make this fact clear.
For here the uncertainty is not as to whether the motion is derived from something, but as to how we ought to distinguish in the thing between the mover and the moved.
It would seem that, in animals, just as in ships and things not naturally organized, what causes motion is separate from what suffers motion and that it is only in this sense that the animal as a whole causes its own motion.
The greatest difficulty, however, is presented by the last case of those that we distinguished.
Where things derive their motion from something else, we distinguished the cases in which the motion is unnatural, and we are left with those that are to be contrasted with the others because the motion is natural.
It is in these cases that difficulty would be experienced in deciding from where the motion is derived, such as in the case of light and heavy things.
When these things are in motion to positions the reverse of those they would properly occupy, their motion is violent; when they are in motion to their proper positions—the light thing up and the heavy thing down—their motion is natural; but, in this latter case, it is no longer evident, as it is when the motion is unnatural, from where their motion is derived.
It is impossible to say that their motion is derived from themselves, which is a characteristic of life and peculiar to living things.
Further, if it were, it would have been in their power to stop themselves (I mean that, for instance, if a thing can cause itself to walk, it can also cause itself not to walk).
And so, since on this supposition, fire itself possesses the power of upward locomotion, it is clear that it should also possess the power of downward locomotion.
Moreover, if things move themselves, it would be unreasonable to suppose that their motion is derived from themselves in only one kind of motion.
Again, how can anything of continuous and naturally connected substance move itself? Insofar as a thing is one and continuous not merely in virtue of contact, it is impassive; it is only insofar as a thing is divided that one part of it is by nature active and another passive.
Therefore, none of the things that we are now considering move themselves (for they are of naturally connected substance), nor does anything else that is continuous. In each case, the mover must be separate from the moved, as we see to be the case with inanimate things when an animate thing moves them.
1021. After revealing his aim, the Philosopher now begins to execute it, namely, to establish that not all things are sometimes in motion and sometimes at rest, but that something is entirely immobile and something always in motion.
The treatment is divided into two parts.
In the first, he shows that the first mover is immobile;
in the second, he shows that the first mobile is always being moved, at “and, further, if there is” (259b32; [1083]).
The first part is divided into two sections:
in the first, he shows the immobility of the first mover from the order of movers and mobiles;
in the second, from the eternity of motion, at “moreover, a conviction” (259a21; [1077]).
The first is divided into two parts:
in the first, he shows that the first mover is immobile;
in the second, he shows that the first mover is eternal, at “since there must always” (258b10; [1069]).
About the first, he does two things:
first, he shows that proving what follows depends on showing that whatever is moved is moved by another;
second, he shows the proposition, at “now, this may come about” (256a4; [1037]).
He had indeed showed above (in the beginning of book 7), by a generic argument based on motion itself, that whatever is moved is moved by another, but because he has now begun to apply motion to mobile things, he here shows that what was previously proved in a universal way is verified universally in all mobiles and movers.
Hence, the first part is divided into two parts:
in the first, he gives a division of movers and mobiles;
in the second, he explains his proposition in individual cases, at “the fact that a thing” (254b24; [1024]).
About the first, he does two things:
first, he divides movers and mobiles;
second, he explains the division, at “thus, in things that” (254b14; [1024]).
1022. He first gives, therefore (254b7), three divisions of movers and mobiles.
The first of these is that, among movers and mobiles some move or are moved accidentally, and some essentially. And he takes accidentally in a wide sense to include what moves or is moved with respect to a part.
Hence in explaining what he means by accidentally, he adds that things cause motion or are moved accidentally in two ways.
First, whatever things are said to cause motion by virtue of being present in things that move are said to cause it per accidens, as when it is said that a musician causes health because a knowledge of music is present in the one who heals; and likewise things are said to be moved per accidens on account of existing in what is being moved either as a placed object in place (such as when we say that a man is being moved because the ship on which he is is being moved) or as an accident in a subject (as when we say that the white is being moved because a body is being moved).
In another way, things are said to move or to be moved per accidens because they move or are moved with respect to a part, as a man is said to strike or be struck because his hand strikes or is struck.
But, when these two per accidens ways of causing motion or being moved are eliminated, things are said to move or to be moved per se—that is, when they are not said to cause motion or be moved by virtue of being in the cause of motion or in what is being moved, or because some part of them causes motion or is moved.
Therefore, leaving out what causes motion or is moved per accidens, he subdivides things that are moved per se into those that are moved by themselves, as are animals, and those moved by others, as are the non-living.
He gives a third division: some things are moved according to nature and some outside of nature.
1023. Then, at “thus, in things that” (254b14), he explains how to discern what is according to nature from what is outside of nature, both in things that are moved by themselves and in things that are moved by something else.
First, with respect to things that are moved by themselves, like animals, he says that they are moved according to nature. And he proves this on the ground that they are moved by an intrinsic principle, and since things whose principle of motion is within are said to be moved by nature, it follows that an animal’s motion by which it moves itself is natural if it is compared to the whole animal, because that motion proceeds from the soul, which is the nature and form of the animal. But, if it be compared to the body, an animal’s motion may be both natural and outside of nature. The difference depends on the type of motion and on the element of which the animal is composed. For if an animal consists of a predominant heavy element, as does the human body, and it is moved upwards, such a motion would be by force with respect to the body; but, if it is moved downward, it will be a motion that is natural to the body. However, if there were animals whose bodies were composed of air, as Platonists held, then the contrary would be true.
Second, he explains how to find forced and natural motions in things that are moved by another. Of these, he says that some are moved according to nature, as fire upward and earth downward; others are moved outside their nature, as earth upward and fire downward, which is a motion by force.
Third, he mentions another type of unnatural motion in animals, namely, those in which the parts of animals are moved outside of nature, their positions and the character of the motion being abnormal. For example, a man’s arms bend facing forward, while his legs bend facing backward, but dogs and horses and the like bend the forelegs facing backward and the hind legs facing forward. If motions contrary to these are made, they will be by force and not according to nature.
1024. Then, at “the fact that a thing” (254b24), he proves that everything that is moved is moved by another.
First, he manifests it in cases that are evident;
second, he manifests in cases about which there is doubt, at “the greatest difficulty” (254b33; [1025]).
Leaving aside things that are moved per accidens, because such things are not moved, but are merely said to be moved when other things are moved, and confining ourselves to those that are moved per se, it is clear, especially in things moved by force and outside of nature, that what is moved is moved by another.
For in the case of things moved by force, it is clear from the very definition of force that they are moved by another. For force, as is said in Ethics 3.1, is that whose principle is from without, with the thing suffering it contributing nothing.
After things that are moved by force, it is clear that what is moved is moved by another if we consider things moved by themselves according to nature, as animals are said to move themselves. For in animals it is clear that something is being moved by something else—but there might yet be a question as to how to distinguish in them the mover and what is being moved. For at first glance, it appears to many that what is true with respect to ships and other artifacts that do not exist according to nature—namely, that the part that causes motion is diverse from the part that is moved—applies to animals, for it seems that the soul, which causes motion, is related to its body, which is moved, as the mariner is related to the ship, as is said in De anima 2.4. In this way, it seems that the whole animal moves itself insofar as one part moves another. But whether the soul is related to the body as a mariner to a ship he leaves to be investigated in De anima. However, the fact that a thing is said to move itself, insofar as one part of it moves and another is moved, will be shown later.
1025. Then, at “the greatest difficulty” (254b33), he explains his proposition in regard to things in which it is more doubtful.
About this, he does three things:
first, he lays out those things in which it is more doubtful that whatever is moved is moved by another, namely, in the heavy and the light, when they are moved according to nature;
second, he shows that they do not move themselves, at “it is impossible” (255a5; [1026]);
third, he shows by what they are moved, at “it is the fact that” (255a18; [1029]).
He says first (254b33) that, since it is in things moved by force, and after them in things that move themselves, that it is especially evident that whatever is being moved is moved by another, the greatest doubt appears to be in the remaining member of the last division, namely, in things that do not move themselves, yet are moved naturally.
The last division to which he refers is that in which he divided things that are moved not by themselves, but by another, dividing them into those that are moved outside of nature and those that are moved according to nature. In these latter, there is doubt as to what moves them. For example, heavy and light objects are moved to their proper places according to nature—that is, the light upwards and the heavy downwards—and into contrary places by force; but the source of their motion when they are moved according to nature is not clear, as it is when they are moved outside of nature.
1026. Then, at “it is impossible” (255a5), he proves with four arguments that these things do not move themselves.
The first is that to move itself pertains to the account of life and is peculiar to living things; for it is through motions and sensations that we distinguish the animate from the inanimate, as is said in De anima 1.2. But it is manifest that the heavy and light as such are not alive, or animate. Therefore, they do not move themselves.
1027. The second argument is given at “further, if it were” (255a7). Things that move themselves can cause themselves to stop, as we see that animals are moved and stop by reason of their appetite. Therefore, if heavy and light things moved themselves with a natural motion, they could cause themselves to stop, just as a person who is the cause of his own walking is likewise that of his own ceasing to walk. But we see that this is false, because the heavy and the light do not stop outside their proper places unless some external cause intervenes to halt their motion. Therefore, they do not move themselves.
But, because someone could say that, although such things are not the cause of their own stopping outside their proper places, yet they are the cause of stopping in their proper places, he adds a third argument at “and so, since on this supposition” (255a9).
It is unreasonable to say that things that move themselves are so moved according to one motion alone and not by many, because what moves itself does not have its motion determined by another, but determines its own motion for itself, such that it determines this motion at one time and that one at another time. Hence, it is in the power of what moves itself to determine for itself this or that motion. Therefore, if heavy and light things moved themselves, it would follow that, if it were in the power of fire to be moved upward, it would also be in its power to be moved downward, which is something we never see occurring unless from an extrinsic cause. Therefore, they do not move themselves.
It should be recognized that these two arguments are probable in respect to what appears in things among us that move themselves, things that are found at one time to be moved with this motion and at another time with that motion, and at yet another time to be at rest. Hence, he does not say that it is impossible, but unreasonable, which is his manner of speaking when he talks of what is probable. For he will show later that, if something is moving itself and it is an entirely immobile mover, it is always being moved and with one motion. Yet that could not be said in regard to heavy and light things, in which there is nothing that is not moved either per se or per accidens, and they are also generated and cease to be.
1028. He gives the fourth argument at “again, how can anything” (255a13). No continuum moves itself; but heavy and light bodies are continua; therefore, neither of these moves itself.
That no continuum moves itself he proves in the following manner. The mover is related to the moved as agent to patient. But, since the agent is contrary to the patient, that which is apt to act must be divided from what is apt to be acted upon. Now, to the extent that things are not in mutual contact but are completely one and continuous in quantity and form, to that extent they can not be acted upon by one another. In this way, therefore, it follows that no continuum moves itself, but the mover must be divided from what is moved, as is evident when non-living things are moved by living things, like the stone by the hand. Hence, too, in animals that move themselves, there is rather a connection of parts than a perfect continuity (for which reason one part can be moved by another), a situation that is not verified in the light and the heavy.