Commentary Aristotle Physics
Book 7. Existence of the First Motion and First Mover
Lecture 1. Whatever is moved must be moved by another
Everything that is in motion must be moved by something. For if it has not the source of its motion in itself, it is evident that it is moved by something other than itself, for there must be something else that moves it.
If, on the other hand, it has the source of its motion in itself, let AB be taken to represent that which is in motion primarily and according to itself and not in virtue of the fact that something belonging to it is in motion.
In the first place, if we were to assume that AB is moved by itself because it is in motion as a whole and is not moved by anything external to itself, this assumption is just as if, supposing that KL is moving LM and is also itself in motion, we were to deny that KM is moved by anything because it is not evident which is the part that is moving it and which the part that is moved.
In the second place, that which is in motion without being moved by anything does not necessarily cease from its motion because something else is at rest, but a thing must be moved by something if the fact of something else having ceased from its motion causes it to be at rest.
Thus, if this is accepted, everything that is in motion must be moved by something. For AB, which has been taken to represent that which is in motion, must be divisible, since everything that is in motion is divisible. Let it be divided, then, at C.
Now, if BC is not in motion, then AB will not be in motion, for if it is, it is clear that AC would be in motion while BC is at rest, and therefore, AB is not being moved per se. But it was hypothesized that AB is in motion primarily and per se.
Therefore, if BC is at rest, AB must rest. But we have agreed that that which is at rest if something else is not in motion must be moved by something. Consequently, everything that is in motion must be moved by something, for that which is in motion will always be divisible, and if the part rests, the whole rests.
884. After discussing motion in itself, the concomitants of motion, and the division of motion into parts in the preceding books, the Philosopher now begins to treat of motion in its relationship to movers and mobiles.
The treatment falls into two parts.
In the first, he shows that there is a first motion and a first mover.
In the second, he investigates the properties of the first motion and of the first mover in book 8, at “it remains to consider” (250b11; [965]).
The first part is divided into two sections.
In the first, he shows that there is a first motion and a first mover.
And because things that belong to one order have some comparison to each other, therefore in the second part, he compares the various types of motion, at “a difficulty may be raised” (248a10; [928]).
About the first, he does three things: first, he mentions the preliminary notes needed for proving the proposition; second, he proves the proposition, at “since everything” (242a15; [891]); third, he proves something he took for granted, at the first mover, not as the cause (243a32; [897]).
885. He thus proposes first (241b24) that everything that is being moved must be moved by some other. In some cases, this is indeed evident, for there are some things that do not possess in themselves the principle of their motion; rather the principle of their motion is from without, as in things that are being moved by force.
Therefore, if there is anything that does not have in itself the principle of its own motion but its principle of motion is from without, it is clear that it is being moved by some other. However, if there is a mobile that does have in itself the principle of its own motion, there could be doubt whether it too is being moved by some other.
Accordingly, he devotes himself to showing that this type of mobile is being moved by some other. Therefore, if it is supposed that such a mobile is not being moved by some other, let AB be a mobile capable of being moved primarily and according to itself and not in the sense that some part of it is being moved;
for then it would not be being moved according to itself, but according to a part. Now, it is necessary that, if something moves itself without having been moved by some other then it is moved primarily and per se; for example, if something is hot not from some other source, it must be primarily and per se hot.
With this in mind, he proceeds to prove his proposition in two ways: first, by excluding the most evident case in which it would appear that something is not being moved by some other; second, by proving directly that nothing can be moved by itself, at “in the second place” (241b34; [886]).
The most evident reason that it seems that something is not being moved by some other is that it is not being moved by something outside itself, but by an intrinsic principle.
He says first (241b24) that to believe that AB is being moved by itself because the whole is being moved—and this without being moved by anything outside of it—is like saying that, when one part of a whole is being moved and another part causes it to be moved, it is moving itself, because it is not evident which part is the mover and which is being moved. Such would be the case if a mobile DEZ is such that one part DE moves the part EZ and it is not seen which part moves the other and which is being moved.
When he speaks of the first mobile AB as being moved as a whole by an intrinsic principle of motion, he means a living body that is, as a whole, being moved by the soul; but, when he speaks of the mobile DEZ, he means some body that is not being moved as a whole, but one bodily part of it is the mover and another the moved. In this latter case, it is evident that what is being moved is being moved by some other.
From this latter case, he wants to prove of a living body that seems to move itself that it too is being moved by some other. For it seems to move itself inasmuch as one part moves another, that is, as the soul moves the body, as will be more fully explained in book 8.
886. Then, at “in the second place” (241b34), he proves directly that whatever is being moved is being moved by some other. This is his argument. Nothing that is being moved by itself rests from its motion on account of some other mobile’s resting. (He takes this as per se known.) From this, he further concludes that, if a mobile rests on account of the rest of another, then the mobile is moved by another. On this ground, he concludes that whatever is being moved is necessarily being moved by some other. And that this follows from the premises he now proves.
That mobile that we have supposed as being moved by itself (that is, AB) must be divisible, for whatever is being moved is divisible, as was proved above. Hence, because it is divisible, nothing prevents it from being divided. Therefore, let it be divided at the point C such that one part of it is BC and the other part AC.
Now, if BC is part of AB, then, when the part BC rests, the entire AB must rest. But if, upon the part resting, the whole does not rest, let us grant that the whole is being moved and one part is at rest.
But, because we have assumed that one part is resting, the whole could not be granted as being moved except by reason of the other part. Therefore, when BC (which is one part) is at rest, the other part AC is being moved. But no whole of which one part only is being moved is being moved primarily and according to itself.
Therefore, AB is not being moved primarily and according to itself, as we originally assumed. Therefore, while BC is at rest, the entire AB must be at rest.
Thus, what is being moved ceases from its motion upon the occasion of something else resting. But we held above that, if something rests and ceases to be moved on the occasion of another’s resting, it is being moved by that other. Therefore, AB is being moved by some other.
The same argument applies to any other mobile, for whatever is being moved is divisible and, for the same reason, if the part rests, the whole rests. Therefore, it is clear that whatever is moved is moved by some other.
887. Many objections are leveled against this argument of Aristotle. Galen objects against the statement that, if just one part of a mobile is being moved and the others are at rest, then the whole is not being moved per se. Galen says this is false, because things that are being moved according to a part are moved per se.
But Galen was deceived by playing on the phrase per se. For sometimes it is taken in opposition to per accidens, and then it is true that what is being moved according to a part is being moved per se, as Galen said. But sometimes per se is taken in opposition both to per accidens and to what is according to a part, and in this sense, something is said to be not only per se but also primarily so. And this is the sense in which it was being used by Aristotle in his proof. That he does so is clear, for after concluding, therefore, AB is not being moved per se, he adds, but it was hypothesized that AB is in motion primarily and per se.
888. But a more serious objection is that of Avicenna, who says against the argument that it proceeds from an impossible assumption, from which the impossible follows, and not from the assumption that something is being moved by itself. For if we assume that a mobile is being moved first and per se, it is natural that it be moved both according to the whole and according to the parts. Therefore, if it is then assumed that a part is at rest, that is the same as assuming what is impossible. And it is from this added assumption that there follows the impossibility that Aristotle deduces, namely, that the whole is not being moved first and per se, as was assumed.
One might obviate this objection by countering that, although it is impossible for a part to rest if you confine yourself to a body of a definite kind—for example, the heaven or fire—yet it is not impossible if you consider the general definition of body, for body as body is not prevented from being at rest or in motion.
However, Avicenna forestalled such a response.
First, for the same reason, it could be said of a whole body that it is not being prevented from resting just because it is a body, just as it is said of the part. Thus, it was superfluous to assume the division of the mobile and the rest of a part in order to prove the proposition.
Second, some propositions are rendered impossible absolutely if the predicate is repugnant to the subject by reason of its specific difference even though it be not repugnant to it by reason of its genus. For it is impossible for man to be non-rational, although he is not prevented from being non-rational from the fact of his being animal. Thus, therefore, it is impossible absolutely that a part of a body moving itself be at rest, for this is contrary to the account of any particular body, even though it be not against the common account of body.
889. With this possible answer rejected, Avicenna solves it in another way. He says that a conditional whose antecedent is impossible and whose consequent is impossible can be true; for example, if man is a horse, he is a non-rational animal. It should be conceded, therefore, that it is an impossible assumption that the mobile be moving itself and yet have the whole or a part of itself at rest, just as it is impossible for fire not to be hot, for fire is the cause of its own heat. Hence this conditional is true: if a part of a mobile moving itself is at rest, the whole is at rest. But Aristotle, if his words are carefully studied, does not speak of the rest of the part, except in a statement that has the force of a conditional. He does not say let BC be at rest, but if BC is at rest, AB must rest, and if the part rests, the whole rests; and from this true conditional, Aristotle proves his proposition.
But, says Averroes, that demonstration is not an absolute demonstration, but one of the type called demonstrating by a sign, or a demonstration quia, in which such conditionals are used.
However, this solution is agreeable in regard to what he says about the truth of a conditional but not in regard to the statement that it is a quia demonstration, for it seems to be a propter quid, because it contains the cause why it is impossible for a mobile to move itself.
To see this, recall that to move oneself is nothing more than to be the cause of one’s own motion. Whatever is the cause of something for itself must possess it primarily, that is, first, because what is first in any group is the cause of what comes after it. Hence fire, the cause of heat for itself and for other things, is the first hot thing. But, in book 6, Aristotle showed that there is no first in motion, whether on the side of time or magnitude or the mobile—for they are all divisible. Therefore, it is impossible to find a first whose motion does not depend on a prior, for the motion of the whole depends on the motions of the parts and is divided into those motions, as was proved in book 6. Aristotle thus shows the cause why no mobile moves itself: there cannot be a first mobile whose motion does not depend on its parts any more than the first being can be a divisible, for the existence of any divisible depends on the parts. Hence, this conditional is true: if the part is not being moved, neither is the whole. Likewise, this conditional is true: if the part does not exist, the whole does not.
890. Hence, even the Platonists, who assumed that some things move themselves, said that no body or divisible thing moves itself; rather, to move itself is a prerogative of a spiritual substance that understands and loves itself (here all operations are being called “motions,” just as Aristotle, in De anima 3.7, calls sensing and understanding by the name of “motions” in the sense that motion is the act of a perfect thing). However, in this book 7, he takes motion as the act of an imperfect thing, a thing existing in potency. It is in this sense of motion that no indivisible is moved, as was proved in book 6 and is here taken for granted. And, so, it is clear that Aristotle, in stating that whatever is moved is moved by some other, and Plato, in stating that some things move themselves, here do not disagree in their opinions, but solely in their words.