Lecture 23 — The cause of the earth's rest is not supporting air

362 Anaximenes and Anaxagoras and Democritus give the flatness of the earth as the cause of its staying still. Thus, they say, it does not cut, but covers like a lid, the air beneath it. This seems to be the way of flat-shaped bodies: for even the wind can scarcely move them because of their power of resistance. The same immobility, they say, is produced by the flatness of the surface which the earth presents to the air which underlies it; while the air, not having room enough to change its place because it is underneath the earth, stays there in a mass, like the water in the case of the water-clock. And they adduce an amount of evidence to prove that air, when cut off and at rest, can bear a considerable weight.

363 Now, first, if the shape of the earth is not flat, its flatness cannot be the cause of its immobility.

364 But in their own account it is rather the size of the earth than its flatness that causes it to remain at rest. For the reason why the air is so closely confined that it cannot find a passage, and therefore stays where it is, is its great amount: and this amount great because the body which isolates it, the earth, is very large. This result, then, will follow, even if the earth is spherical, so long as it retains its size. So far as their arguments go, the earth will still be at rest.

365 In general, our quarrel with those who speak of movement in this way cannot be confined to the parts; it concerns the whole universe. One must decide at the outset whether bodies have a natural movement or not, whether there is no natural but only constrained movement. Seeing, however, that we have already decided this matter to the best of our ability, we are entitled to treat our results as representing fact. Bodies, we say, which have no natural movement, have no constrained movement; and where there is no natural and no constrained movement there will be no movement at all. This is a conclusion, the necessity of which we have already decided, and we have seen further that rest also will be inconceivable, since rest, like movement, is either natural or constrained. But if there is any natural movement, constraint will not be the sole principle of motion or of rest. If, then, it is by constraint that the earth now keeps its place, the so-called 'whirling' movement by which its parts came together at the center was also constrained. (The form of causation supposed they all borrow from observations of liquids and of air, in which the larger and heavier bodies always move to the center of the whirl. This is thought by all those who try to generate the heavens to explain why the earth came together at the center.

504. Having presented two solutions, the first of which derived the cause of the earth's rest from the infinitude of the nether part of the earth, while the second derived it from water being under the earth, he here gives a third solution in which the cause is ascribed to air supporting the earth.

First he presents the solution;

secondly, he refutes it, at now, first (294b23; [505]).

He says therefore first that Anaximenes and Anaxagoras and Democritus gave the broadness of the earth as the cause of its state of rest. That is why it happens that the earth does not cut the air beneath it, but rides upon it. Now this is indeed what seems to be done by bodies purposely made wide in order to resist air and wind: for such very wide bodies do not seem to be easily moved by winds because they resist them along their entire width. This, too, the earth, because of its width, seems to do with respect to the air under it, i.e., instead of cleaving it, the earth resists and compresses it. And, since the air has no place to move in order not to be under the earth, because of the earth's width, the vast amount of pressed air under the earth is sufficient to uphold it, like the water in the case of the water-clock. For if a vessel with a small opening in the top, which is stopped up, and in the side another such opening not so stopped up, be suddenly submerged in water, the air imprisoned within, having no place to escape to, will keep water from entering. Similarly, the air residing under the earth, being pressed down by it and unable to escape, prevents the earth from falling. And they adduce many arguments, i.e., sensible signs, to show that air imprisoned and at rest, i.e., which cannot escape in any direction, sustains a great heaviness. And this is particularly true of inflated skins, which can support a great weight.

505. Then, at now, first (294b23), he refutes this solution with three arguments.

The first of these is that this solution assumes the earth to be broad. But as we shall see later, this is false. Hence, if the earth is not broad but spherical, it will follow that its width will not account for its state of rest, as they alleged.

506. He gives the second argument, at but in their own account (294b24). And he says that although they assign the earth's width as the cause of its rest, yet, according to that from which they proceed, it seems that it is the earth's size rather than its width that causes it to rest. For they say that the air, having no place to go because it is prevented by the earth, remains because there is so much of it, and because of this holds up the earth. But the reason why a vast amount of air is kept in confinement by the earth is that it is compressed by the enormous magnitude of the earth. Hence it seems that the same argument would hold if the earth were assumed to be spherical and of such a great size that it could confine the same amount of air—for then, too, the air and the earth would remain, according to the reason they give.

507. He gives the third argument at in general, our quarrel (294b30), and he says that against those who thus speak about the motion and rest of natural bodies there arises a doubt not about the part, i.e., concerning any particular body, such as earth or water, but concerning the entire universe and every natural body. For the first thing that seems to have to be determined in such problems is whether bodies have some natural motion, or none, and whether, if they do not have a natural motion, they can have violent [compulsory] motion. And because this question has already been determined, namely, in Book I, we must now use as in existence, i.e., as true, all that we have previously established on this point according to the power that was then present in our talent. For it has been shown above that, if there is no natural motion of bodies, neither can there be any compulsory motion thereof, because the compulsory is, as it were, a departure from that which is according to nature, as has been had above. But if there is neither a natural nor a compulsory motion of bodies, it follows that nothing is in motion at all—that this is so has been previously determined. To this must be added, according to what has been previously determined, that for the same reason nothing can be at rest—for just as there is motion which is natural and compulsory, so too, rest which is natural and compulsory. But if natural motion does exist, then there will not be compulsory motion alone nor compulsory rest alone—because, into whatever place a thing is moved naturally, there it also rests naturally.

Having prefaced these points as principles, he argues to his proposition and concludes from these premises that, if the earth's rest in the middle is not natural but compulsory, it follows that its motion to the middle is not natural. but is due to compulsion exercised by the circumgyration of the heaven. For all who maintain that the earth rests in the middle through compulsion assign this cause of the motion of the earth to the middle, namely, the circumgyration of the heaven, basing themselves on the consideration of what happens in the case of liquids, and also of air, namely, that the larger and heavier masses are gathered in the middle as though more violently repelled by the violence of the gyration. Hence all who posit a world that began by generation say that the earth came to the middle for the reason stated, i.e., on account of the compulsion induced by the heaven's circumgyration. They thus take from the earth both natural rest and natural motion. But this is unacceptable, because it follows, according to what has been said, that in their entirety natural bodies are neither in motion nor at rest.