Commentary Aristotle Physics
This motion of projection is by some called antiperistasis, that is, contra-resistance, on the ground that the surrounding air being set in motion somehow moves the projectile, as was said in book 4. However, the problem under discussion can be solved in no other way than the way mentioned.
For if the contra-resistance of the air is the cause of the projection, it follows that all the elements involved are moving and being moved simultaneously—that is, that the entire air is simultaneously acting and being acted upon—and, consequently, that all would cease simultaneously. But this is evidently false. For we see some one thing being moved continuously no matter what moves it. And I say this because it does not have one and the same determinate mover, but diverse movers.
Lecture 23. The first mover cannot have magnitude
We proceed from the positions
(1) that there must be continuous motion in the world of things;
(2) that this is a single motion;
(3) that a single motion must be a motion of a magnitude (for that which is without magnitude cannot be in motion);
(4) that the magnitude must be a single magnitude moved by a single mover (for otherwise, there will not be continuous motion, but a consecutive series of separate motions);
and (5) that, if the motion is a single thing, it is either itself in motion or itself unmoved.
If, then, it is in motion, it will have to be subject to the same conditions as that which it moves: it will itself be in process of change and, in being so, will also have to be moved by something.
Thus, we have a series that must come to an end, and a point will be reached at which motion is imparted by something that is unmoved.
Thus, we have a mover that has no need to change along with that which it moves, but will be able to cause motion always (for the causing of motion under these conditions involves no effort).
And this motion alone is regular, or at least it is so in a higher degree than any other, since the mover is never subject to any change. So, too, in order that the motion may continue to be of the same character, the moved must not be subject to change in respect of its relation to the mover.
Moreover, the mover must occupy either the center or the circumference, since these are the first principles from which a sphere is derived.
But the things nearest the mover are those whose motion is quickest, and in this case, it is the motion of the circumference that is the quickest: therefore, the mover occupies the circumference.
There is a further difficulty in supposing it to be possible for anything that is in motion to cause motion continuously and not merely in the way in which it is caused by something repeatedly pushing (in which case the continuity amounts to no more than successiveness).
Either such a mover must itself continue to push or pull or perform both these actions or else the action must be taken up by something else and be passed on from one mover to another (the process that we described before as occurring in the case of things thrown, since the air or the water, being divisible, is a mover only in virtue of the fact that different parts of the air are moved one after another).
And, in either case, the motion cannot be a single motion, but only a consecutive series of motions.
The only continuous motion, then, is that which is caused by the unmoved mover, and this motion is continuous, because the mover remains always invariable such that its relation to that which it moves remains also invariable and continuous.
Now that these points are settled, it is clear that the first unmoved mover cannot have any magnitude. For if it has magnitude, this must be either a finite or an infinite magnitude. Now, we have already proved in our course on physics that there cannot be an infinite magnitude, and we have now proved that it is impossible for a finite magnitude to have an infinite force, and also that it is impossible for a thing to be moved by a finite magnitude during an infinite time. But the first mover causes a motion that is eternal and does cause it during an infinite time. It is clear, therefore, that the first mover is indivisible, without parts, and without magnitude.
1164. Having resolved the doubt he raised about the motion of projectiles, from the solution of which he concluded that a motion involving a number of movers is not one continuous motion, the Philosopher now turns to his main task, namely, to prove that the first mover is one.
About this, he does two things:
first, he states his proposition;
second, he raises a doubt and solves it, at “there is a further difficulty” (267b9; [1170]). About the first, he does three things: first, he proves the unity of the first mover through the continuity of motion;
second, he shows how a continuous motion comes from one mover, at “if, then, it is in motion” (267a25; [1166]);
third, he shows where the principle of a continuous motion is, at “moreover, the mover” (267b6; [1168]).
1165. That there must be one mover he proves (267a21) through the continuity of motion, taking what he had previously proved, namely, that some continuous motion must always exist. But a continuous motion is one, as was said in book 5. Therefore, there must always be some one motion. But, for a motion to be one, it must be of one moved magnitude (because something not able to be divided into parts cannot be moved, as was proved in book 6) and it must be moved by one mover. For if the mobiles are diverse or the movers are diverse, a motion will not be one, and consequently, not continuous. Rather, it will be one motion divided from another—on account of the division of the mobile or mover—and will have consecutive motions. It is necessary, therefore, that the mover be one and that it be either a moved mover or a mover that is immovable.
1166. Then, at “if, then, it is in motion” (267a25), he shows how, from one mover, there can be a continuous motion.
About this, he does two things:
first, he shows how, from one mover, there can be a motion ever continuous;
second, he shows how it is regular, at “and this motion alone” (267b3; [1167]).
He says, therefore (267a25), that one motion from one mover is, as has been said, either from a moved mover or an unmoved mover. If it is the former, it follows that it is moved by something, as was proved above. But this cannot go on to infinity, as was proved above. Therefore, the series of movers and mobiles must stop, and a first mobile moved by an immobile mover be reached, which mover does not move of necessity, because it is not moved by another. For whatever is moved by another moves of necessity, to the extent that necessity is imposed upon it by its mover. And, because it is changed from its disposition, it cannot cause motion that is always uniform, for its disposition varies.
But nothing other imposes necessity on an unmoved mover, nor does its disposition vary. Hence, it does not act of necessity, but it can move always, because to move thus—namely, without change of self—is unwearying. For fatigue occurs to some movers in moving because they are also simultaneously moved themselves, and from fatigue, it occurs that they cannot always act as movers. Hence, it remains that an unmoved mover can move with a perpetual continuous motion.
1167. And, because perfect continuity and unity of motion require that a motion be regular and uniform, as was had in book 5, therefore, at “and this motion alone” (267b3), he shows that a motion from an immobile mover is regular.
And he says that either solely the motion from an immobile mover is regular or, if any others are regular also, the former is the most regular. Now, he uses this disjunction because the disposition of a moved mover sometimes remains the same for some time without variation (at least as far as any sensible perception is concerned), and accordingly, such a mover seems for a time to cause a uniform motion. But that which is always such moves above all with a uniform motion, since such a mover is subject to no change whatsoever. He says this in order to show that there are some movers that are not moved with the same kind of motion as they cause, as a heavenly body is not moved by the motion of alteration, but by some other, namely, local motion. But the first mover, being utterly immobile, is moved by no change.
In order that a motion be regular and uniform, it is required that the mover be wholly immobile; besides that, in order that the motion be similar, that is, uniform, it is required that what is moved not undergo any change other than that which the immobile mover causes in it, as a heavenly body is moved with local motion by an immobile mover and, beyond that, has no other change. For if it were altered, its disposition to the motion would not remain constant and, consequently, the motion would not be uniform.