Lecture 10 — How a thing moves itself

Now, everything that is in motion must be infinitely divisible, for it has been shown already about universals of nature that everything that is essentially in motion is continuous. Now, it is impossible that that which moves itself should in its entirety move itself.

For then, while being specifically one and indivisible, it would as a whole both undergo and cause the same locomotion or alteration, and thus it would at the same time be both teaching and being taught, or both restoring to health and being restored to the same health.

Moreover, we have established the fact that it is the movable that is moved; and this is potentially in motion, not actually, but the potential is in process to act, and motion is an imperfect act of the movable.

The mover, on the other hand, is already in activity; for instance, it is that which is hot that produces heat—in fact, that which produces the form is always something that possesses it.

Consequently, if a thing can move itself as a whole, the same thing in respect of the same thing may be at the same time both hot and not hot, and this is so, too, in every other case where the mover must be univocal with the moved.

Therefore, when a thing moves itself, it is one part of it that is the mover and another part that is moved.

But it is not self-moving in the sense that each of the two parts is moved by the other part. The following considerations make this evident.

In the first place, if each of the two parts is to move the other, there will be no first mover.

If a thing is moved by a series of movers, that which is earlier in the series is more the cause of its being moved than that which comes next and will be more truly the mover.

For we found that there are two kinds of mover, that which is itself moved by something else and that which derives its motion from itself; and that which is further from the thing that is moved is nearer to the principle of motion than that which is intermediate.

In the second place, it is not necessary for a mover to be moved except by itself according to accident, such that the other part can move it in return only accidentally.

I take, then, the possible case of its not moving it; then there will be a part that is moved and a part that is an unmoved mover.

In the third place, there is no necessity for the mover to be moved in return: on the contrary, the necessity that there should always be motion makes it necessary that there should be some mover that is either unmoved or moved by itself.

In the fourth place, we should then have a thing undergoing the same motion that it is causing—that which is producing heat being heated.

But, as a matter of fact, that which primarily moves itself cannot contain either a single part that moves itself or a number of parts each of which moves itself.

For if the whole is moved by itself, it must be moved either by some part of itself or as a whole by itself as a whole.

If, then, it is moved in virtue of some part of it being moved by that part itself, it is this part that will be the primary self-mover, since, if this part is separated from the whole, the part will still move itself, but the whole will do so no longer.

If, on the other hand, the whole is moved by itself as a whole, it must be accidentally that the parts move themselves; therefore, their self-motion not being necessary, we may take the case of their not being moved by themselves.

Therefore, in the whole of the thing, we may distinguish that which imparts motion without itself being moved and that which is moved; for only in this way is it possible for a thing to be self-moved.

Further, if the whole moves itself, we may distinguish in it that which imparts the motion and that which is moved, so while we say that AB is moved by itself, we may also say that it is moved by A.

1050. After showing that, in mobiles and movers, there is no going on to infinity, but that a first is reached that is either immobile or self-moving, the Philosopher now shows that, even if a first that moves itself is reached, it is nevertheless necessary to come to a first that is immobile.

This treatment is divided into three parts.

In the first, he shows that what moves itself is divided into two parts, one of which is mover and the other moved;

in the second, he shows how these parts are mutually related, at and, since what imparts (258a5; [1062]);

in the third, he concludes that it is necessary to come to a first that is immobile, at from what has been said (258b4; [1068]).

About the first, he does two things:

first, he shows that, in a thing that moves itself, one part is mover and the other is moved, because a whole cannot move its whole self;

second, he rejects other ways in which a thing that moves itself might be thought to do so, at but it is not (257b13; [1054]).

About the first, he does three things:

first, he proposes that what moves itself does not totally move itself as a whole;

second, he proves the proposition, at for then, while being (257b2; [1052]);

third, he argues for the main conclusion intended, at therefore, when a thing (257b12; [1053]).

1051. Because whole and part have no place except in things that are divisible, Aristotle concludes from what he had proved in book 6 (240b8), first, that whatever is moved is necessarily divisible into parts that are always further divisible—for this pertains to the very account of a continuum. Now, whatever is being moved is a continuum if it is being moved per se (for it is not impossible for an indivisible, such as a point or whiteness, to be moved per accidens).

And this was shown previously in book 6; (Lect. 5, n. 796.) all the statements made prior to book 8 he calls universals of nature, for in book 8, he begins to apply to things the statements he previously made about motion in common. Accordingly, since what is moved is divisible, a whole and a part can be found in everything that is being moved. If, therefore, there is anything that moves itself, we shall be able to take a whole and a part in it; but a whole cannot move its whole self, that is, in its entirety move itself.

1052. Then, at for then, while being (257b2), he proves his proposition with two arguments.

The first is this. The motion of a thing that moves itself at one time and in one motion is numerically one; if, therefore, a thing should move itself in such a way that the whole moves the whole, it will follow that one and the same will be mover and moved with respect to one and the same motion, whether it be local motion or alteration. But this seems unsuitable, for mover and moved are mutually opposite, and opposites cannot exist in the same thing with respect to the same. It is, therefore, not possible that one thing be mover and moved with respect to the same motion.

For when something is at once moving and being moved, the motion according to which it moves is different from the one according to which it is being moved, as when a stick, moved by the hand, moves a stone, the motion of the stick is numerically different from the motion of the stone.

Accordingly, it will follow further that someone will be both teaching and taught at the same time with respect to one and the same knowable thing, and, similarly, that someone will heal and be healed with respect to one and the same numerical health.

1053. He gives the second argument at moreover, we have established (257b6), which is this. It has been determined in book 3 (Lect. 2, n. 285. Cf. Aristotle, Physics 3.2, 202a7; St. Thomas, Commentary on Physics, bk. 3, lect. 4, n. 302.) that what is being moved is a mobile (that is, something existing in a state of potency), since what is being moved is being moved precisely because it is in potency and not in act, for a thing is considered to be in motion when, being in potency, it is tending toward act.

However, that which is being moved is not in potency in such a way that it is in no way in act, because the very motion is a kind of act of the mobile precisely as being moved; but it is an imperfect act, being the act of the mobile inasmuch as it is still in potency.

But what causes motion is already in act, for what is in potency is not reduced to act except by something in act, and this is the mover. For example, the hot causes heat, and what generates has the form to be generated, as one who has the human form generates a man, and so on for other things. If, therefore, the whole moves its whole self, it follows that the same thing is, with respect to the same, at once hot and not hot, for insofar as it moves, it will be hot in act; insofar as it is moved, it will be hot in potency.

The same is true in all other cases in which the mover is univocal, that is, agreeing in name and account with the thing moved, as when the hot makes the hot and a man generates a man.

And he says this because there are some agents that are not univocal and that do not agree in name and account with their effects, as the sun generates a man. In such agents, nevertheless, even though they do not possess the form of the effect according to the same account, they do so in a higher and more universal sense. Consequently, it is universally true that the mover is somehow actually what the mobile is potentially. If, therefore, the whole moves its whole self, it follows that the same thing is at once in potency and in act—which is impossible.

From this, he concludes (257b12) the main proposition that, with respect to a thing that moves itself, one part is mover and the other part moved.

1054. Then, when he says, but it is not (257b13), he rejects certain ways in which someone might suppose that a thing moves itself.

First, he shows that, with respect to a thing that moves itself, both parts are not moved by each other;

second, he shows that, with respect to a thing that moves itself, one part does not move itself, at, but, as a matter (257b26; [1059]).

About the first, he does two things:

first, he proposes what he intends;

second, he proves his proposition, at in the first place (257b15; [1055]).

He says therefore, first (257b13), that it is clear from what follows that a thing cannot move itself in such a way that each part is moved by the other—for example, if AB moves itself, that A move B, and B move A.

1055. Then, at in the first place (257b15), he proves the proposition with four arguments. And it should be noted that, for this conclusion, he reuses the reasons previously given to show that not every mover is being moved by another. (Lect. 9.)

Hence, from the foregoing, he here collects four abridged arguments.

The first of these he takes from the first argument presented above (Lect. 9, nn. 4–5.) in a double order (ascending and descending) to show that the process of one thing being moved by another does not go on forever to infinity, because then there would be no first mover, from whose non-existence would follow the non-existence of all coming after it. Hence, in this place, too, the Philosopher argues that the outcome is likewise unacceptable.

For he says that, if both parts in the first thing moved, which moves itself per our supposition, are reciprocally being moved by each other, it will follow that there is no first mover. This follows because, as was said above, the prior mover is more the cause of motion and moves more than the subsequent mover. (Lect. 9, nn. 1038–39.)

And this was proved above on the ground that something causes motion in two ways.

In one way, something moves by being moved by another, as a stick moves a stone because it is being moved by the hand, and this is a second mover.

In another way, something moves by being moved of itself, as a man moves, and this is a condition of a first mover. Now, what causes motion independently of being moved by another is farther removed from the last thing moved and nearer to the first mover than an intermediate that causes motion because it is moved by another.

This argument should be formulated in the following way. If both parts of a thing that moves itself move each other reciprocally, one is no more the cause of motion than the other. But the first mover is more a cause of motion than a second mover; therefore, neither of the parts will be a first mover. Now, this is unacceptable, since it would then follow that what is moved of itself would be no nearer to the first principle of motion (whose existence would thereby be rejected) than what is moved by another, whereas it was proved above that a mover that moves itself is first in the genus of mobiles. (Lect. 9, n. 1049.) Therefore, it is not true that both parts of a thing that moves itself are moved by each other.

1056. Then, at in the second place (257b20), he presents two arguments for the same, taken from one he used above (Lect. 9, n. 1043.) when he showed that not every mover is being moved, in the sense that being moved is found per accidens in the mover.

In this argument, he drew two conclusions above:

first, that a mover can happen not to be moved;

second, that motion is not eternal.

In the light of these two conclusions, he now forms two arguments.

For he says first of all that it is not necessary for a mover to be moved except by itself according to accident, the sense of this being that, unless the first mover be taken as being moved by itself, it will not also be necessary that the first mover be moved according to an accident, as some posited that every mover is being moved, but that its being moved is in it per accidens.

When, therefore, it is supposed that, of a thing that moves itself, the part causing motion is equally being moved by the other, this will be only per accidens. But, as we conceded above, whatever is per accidens is able not to be; therefore, it is possible for the part which causes motion not to be moved. Thus, therefore, it remains that, of a thing that moves itself, one part is moved and the other causes motion and is not moved.

1057. Then, at in the third place (257b23), he gives another argument corresponding to the second conclusion that he inferred above, namely, that it follows that motion does not always exist. Here, however, he argues in reverse order. If it is necessary that motion always exist, it is not necessary that a mover, when it causes motion, be moved, but it is necessary either that the mover be immobile or that it be moved by itself.

The reason for this conditional is apparent from an argument given above. If a mover does not cause motion unless it is being moved, and if being moved is only in it per accidens, it follows that it can happen not to be moved.

Consequently, it can happen also not to cause motion, and as a result, there will be no motion. But motion was proved to be eternal. (Lect. 1ff.) Therefore, it is not necessary for a mover to be moved when it is causing motion, Consequently, it is not true that each part of a thing that moves itself is moved by the other.

1058. Then, at in the fourth place (257b25), he presents the fourth argument, which is taken from the argument previously given (Lect. 9, n. 1046.) to prove that it is not essential to a mover that it be moved, because it would follow that we must come to this: a mover would be being moved by the same motion that it is causing, as explained above.

And now, abridging this argument, he says that, if each part is being moved by the other, it will follow that it causes motion and is being moved with respect to the same motion. Hence, it follows that the heater is heated—which is impossible.

Now, the reason that it follows that the same thing is causing motion and being moved with respect to the same motion when it is posited that each part of a thing that moves itself is moved by the other is that there is just one motion in the thing that moves itself, and it is according to that motion that the part causing motion will itself have to be moved.

1059. Then, at but, as a matter of fact (257b26), he excludes another way, namely, the supposition that the part of a thing that moves itself does not move itself.

First, he proposes what he intends;

second, he proves his proposition, at for if the whole (257b28; [1060]).

He thus says first that, if something that is first moving itself be assumed, it cannot be said either that one part of it moves itself or that a number of parts do so in such a way that each of them moves itself.

1060. Then, at for if the whole (257b28), he proves this with two arguments.

The first is that, if the whole is being moved by itself, this belongs to it either by reason of a part that is being moved by itself or by reason of the whole.

If it belongs to it by reason of its part, then that part will be a first mover that moves itself, because that part separated from the whole will move itself, but then the whole will no longer be a first mover of itself, as was supposed.

But, if it be said that the whole moves itself by reason of the whole, then it will be only per accidens that some parts move themselves. But what is per accidens is not necessary. Therefore, in the mover that first moves itself, it is most important to presume that the parts are not moved by themselves. Therefore, one part of the first mover that moves itself will cause motion, since it is immobile, and the other will be moved. For those are the only two ways in which it is possible that a part that causes motion could be moved, namely, either because that part would be moved by another part that it moves or because that part would move itself.

Hence, it should be noticed that Aristotle, in excluding these two ways, intends to conclude that, in a thing that moves itself, the part that causes motion is immobile, but not that what moves itself is divided into two parts, one of which causes motion and the other of which is moved; for this had been sufficiently concluded when he first proved that the whole does not move itself as a whole.

Accordingly, it is clear that it was not necessary that Aristotle introduce a division of five members, as some claimed:

one member of which is that the whole moves the whole;

the second that the whole moves a part;

the third that a part moves the whole;

the fourth that two parts mutually move one another;

the fifth that one part is a mover and the other moved.

For if the whole does not move the whole, it follows for the same reason that the whole does not move the part, nor the part the whole, for in either case, it would follow that a moved part would be moving itself. Hence, the fact that the whole does not move the whole suffices for concluding that one part is a mover and the other is moved. But, in order to conclude that the part that causes motion is not moved, he proves two other things: that the part causing motion is not moved by a moved part and that it is not moved by itself.

1061. And, to prove this last point, he presents a second argument, at further, if the whole moves itself (258a3). If it be granted that the motion-causing part of a self-mover moves itself as a whole, it follows, through what was proved above, that a part of that part causes motion and the other part is moved.

For it has been already proved above that a whole does not move itself in any other way than by one of its parts causing motion and the other being moved. So, let AB be the motion-causing part of a thing that moves itself; then, by the previous argument, it follows that one part of it is a mover, namely A, and the other part, namely B, is moved.

Therefore, if AB as a whole moves itself as a whole, as you say, it follows that the same thing would be moved by two movers—by the whole AB and by the part A—which is impossible. It remains, therefore, that the motion-causing part of a thing that moves itself is entirely immobile.