Lecture 24 — Earth's rest not from gyration of the heaven

366 They then seek a reason for its staying there; and some say, in the manner explained, that the reason is its size and flatness, others, with Empedocles, that the motion of the heavens, moving about it at a higher speed, prevents movement of the earth, as the water in a cup, when the cup is given a circular motion, though it is often underneath the bronze, is for this same reason prevented from moving with the downward movement which is natural to it.)

367 But suppose both the 'whirl' and its flatness (the air beneath being withdrawn) cease to prevent the earth's motion, where will the earth move to then? Its movement to the center was constrained, and its rest at the center is due to constraint; but there must be some motion which is natural to it. Will this be upward motion or downward or what? It must have some motion; and if upward and downward motion are alike to it, and the air above the earth does not prevent upward movement, then no more could air below it prevent downward movement. For the same cause must necessarily have the same effect on the same thing.

368 Further, against Empedocles there is another point which might be made. When the elements were separated off by Hate, what caused the earth to keep its place? Surely the 'whirl' cannot have been then also the cause.

369 It is absurd too not to perceive that, while the whirling movement may have been responsible for the original coming together of the art of earth at the center, the question remains, why now do all heavy bodies move to the earth. For the whirl surely does not come near us.

370 Why, again, does fire move upward? Not, surely, because of the whirl. But if fire is naturally such as to move in a certain direction, clearly the same may be supposed to hold of earth.

371 Again, it cannot be the whirl which determines the heavy and the light. Rather that movement caused the pre-existent heavy and light things to go to the middle and stay on the surface respectively. Thus, before ever the whirl began, heavy and light existed; and what can have been the ground of their distinction, or the manner and direction of their natural movements? In the infinite chaos there can have been neither above nor below, and it is by these that heavy and light are determined. It is to these causes that most writers pay attention:

508. Having presented three reasons for the earth's rest, taken on the part of the lower bodies, namely, earth,. water and air, he here presents other arguments taken on the part of the heavenly body.

First he gives the fourth reason for the earth's rest, namely, the one which Empedocles gave;

secondly, he refutes it, at but suppose both the 'whirl' (295a21; [509]).

He says therefore first (295a14) that, since all the philosophers who assert than the world was generated, assign, as the cause of the earth's movement to the middle, the violence of the heaven's circumgyration, they also inquire into the reason why the earth rests in the middle. Some say that the cause of this is the earth's width and size, as was said above; but others, such as Empedocles, assert that the motion of the heaven around the earth is so swift that it keeps the earth from being moved. And they give the example of water contained in cyathi, i.e., certain bronze vessels. For if that vessel be given a quite rapid circular motion and there is an opening in some part of the bronze vessel, then, after the vessel is spun a number of times, the water will descend to the bottom of the bronze vessel where the opening is; yet it will not fall out of the vessel (as it normally should according to its natural inclination) for the same reason, namely, because it is prevented by the speed of the vessel's motion in such a way that the water is seized by the motion of the vessel before it can fall. With a like argument, they say that the earth. is prevented by the speed of the heaven's motion from being able to fall downwards.

509. Then, at but suppose both the 'whirl' (295a21) he rejects this reason.

First with respect to the earth's rest;

secondly, with respect to its motion, at it is absurd (295a32; [511]).

In regard to the first he does two things:

first he rejects the cause of the earth's rest in general, both with respect to those who assign the width or size of the earth as the cause of its rest, and also with respect to Empedocles;

secondly, he specifically rejects this theory as far as it pertains to Empedocles, who assigned the speed of the heaven's motion as the cause of the earth's rest, at adhuc autem ad Empedoclem.

He says therefore first (295a21) that, since the aforesaid philosophers declare that the cause of the earth's rest is the motion of the heaven, or the earths width which corners the lower air so that it cannot escape, then it seems necessary that, if the heaven's gyration should not. restrain the earth from moving; and if the earth's width should not restrain it by confining the air, but the air should freely come and go, the earth would be carried somewhere—since now, the causes of rest removed, it would have to move. But it does not seem, according to their theory, that it would be carried to the middle according to its nature. If, then, as they say, the earth is carried to the middle by compulsion, then it would have to rest in the middle by compulsion, which they also posit. However, the earth has to have some natural motion after all compulsion ceases—for all bodies have some motion that is natural, as has been said above. It remains, therefore, to investigate in which direction the earth would be moved naturally, if all compulsion ceased—whether upward, or downward, or in some other direction, e.g., to the right or to the left, for it certainly must have some natural motion. Nor can a reason be found for it to be naturally moved in any other direction except down and to the middle, as is evident from the motion of particles of earth, which are naturally moved in no other direction. Consequently, in assigning compulsion as the cause of the earth's rest in the middle, they make a poor choice. But if they say that the natural motion of the earth does not incline it downward any more than upward, it seems to follow that, just as the air which is atop the earth does not keep it from being moved upward, so too neither will the air under the earth keep it from being moved downward, whether on account of the air's being confined by the earth's width or on account of its being revolved by the motion of the heaven—because, in dealing with the same things as to the same effects, the same causes must be posited.

510. Then, at adhuc autem ad Empedoclem (295a29), he specifically disproves Empedocles' solution. Here it should be kept in mind that Empedocles posited four material elements and two movers, namely, friendship and strife, which, by associating and disassociating the elements, are the cause of the generation and corruption of the world and all things in the world. Aristotle says, therefore, that someone can bring against Empedocles the following difficulty: when the elements were separated from one another by strife, earth had to be at rest—because it did not join itself to the other elements so long as strife ruled. Therefore, one should ask what caused earth to be at rest at that time. One cannot give as a cause the gyration of the heaven, because the heaven had not yet been generated. It seems, therefore, that one can in no way say that the gyration of the heaven is the cause of the earth's rest.

But there seems to be some problem about this argument. For strife seems to be the cause of the generation of the world by disassociating the elements from one another, while friendship seems to be the cause of its corruption by uniting the elements into one chaos. Hence, one seems to have at present the rule of strife, since the elements are disassociated from one another. Accordingly, Alexander expounds these words in the following manner: when the elements were apart, means, not from one another, but from strife, i.e., at the time when strife was disassociated from the elements, namely, at the time when friendship ruled. But because this explanation seems extorted, it should be explained as Simplicius says, as follows: when the elements were apart, i.e., from one another, and this from strife, i.e., on account of strife. For it must be remembered that Empedocles explained the generation of the world, not in teems of strife alone, but with an admixture of friendship as well. And, as he [Simplicius] proves from the words of Empedocles, it is from the dominance of friendship that the circumgyration of the heaven comes, since the motion of the heaven masses, as it were, all things into one. Therefore, it is suitable for Aristotle to ask: What was the cause of the earth's rest before the gyration of the world was, according to Empedocles, caused by friendship?

511. Then, at it is absurd (295a32) he rejects the reason which they assign in general for the motion of the earth. This he does with three arguments.

With respect to the first of which, he says that it is unacceptable not to consider why it is that if before, when the world was being generated, the parts of earth were carried to the middle on account of the heaven's gyration, now, when such a cause can no longer be assigned, we still see all heavy things carried to the middle. For the heaven's gyration rotates at once fire and the upper region of air, but not this lower region of air—and, consequently, this gyration does not reach as far as us. Yet we observe that heavy things are carried to the middle in this air near us. Therefore, the gyration of the heaven ought not be posited as the cause of heavy things' being moved to the middle—because if the cause is taken away, so too the effect.

512. At why, again, does fire (295b1) he presents the second argument. And he says that we must consider why it is that fire is borne upwards. For it cannot be said that this s due to the heaven's gyration— since the example they give does not extend this far. However, if it is on account of its natural inclination that fire is borne to some certain place, then it is plain that the same should be thought of earth, which has contrariety to fire, as has been said above—for the motions of contraries are themselves contrary, and if one of the contraries is natural, the other too must be natural, as has been said.

513. He presents the third reason at again, it cannot be the whirl (295b3) and says that if anyone considers their words and examples, it does not seem that one should say that the heavy is distinguished from the light among bodies on account of the heaven's gyration, but that, if we presuppose the distinction between the heavy and the light, then some, namely, the heavy, move to the middle, while others, namely, the light, try to move upward, insofar as they are repelled from the middle place by the heavy bodies carried there. Consequently, it is only by accident that the heaven's gyration causes the upward motion of fire. But that the gyration of the heaven does not distinguish the heavy from the light, but pre-supposes their distinction, can be seen from the example they adduce—for it: the gyration of air and liquids, things that are already heavy are carried to the middle. Consequently, before there was a gyration of the heaven, there existed heavy and light. These were distinguished according to something, namely, their aptitude to be moved in some way and to some certain place—for a thing is called "heavy" or "light" on account of an inclination to some certain local motion. Consequently, gyration is not the reason why light things are moved upward or heavy things downward.

Now these could distinguish heavy and light and their places, which are above and below, because they did not posit the universe to be infinite. For it is not impossible to distinguish up and down, if they distinguish heavy and light. And because some assumed an infinite universe, namely, Anaximenes and Xenophanes, Aristotle therefore says significantly that many of them, but not all, were worn out, i.e., accustomed to, and practiced in, the matter of the causes of the motion and rest of heavy and light things.