Others say the earth rests upon water.This, indeed, is the oldest theory that has been preserved, and is attributed to Thales of Miletus.It was supposed to stay still because it floated like wood and other similar substances, which are so constituted as to rest upon but not upon air.As if the same account had not to be given of the water which carries the earth as of the earth itself! It is not the nature of water, any more than of earth, to stay in mid-air: it must have something to rest upon.Again, as air is lighter than water, so is water than earth: how then can they think that the naturally lighter substance lies below the heavier? Again, if the earth as a whole is capable of floating upon water, that must obviously be the case with any part of it.But observation shows that this is not the case.Any piece of earth goes to the bottom, the quicker the larger it is.These thinkers seem to push their inquiries some way into the problem, but not so far as they might.It is what we are all inclined to do, to direct our inquiry not by the matter itself, but by the views of our opponents: and even when interrogating oneself one pushes the inquiry only to the point at which one can no longer offer any opposition.Hence a good inquirer will be one who is ready in bringing forward the objections proper to the genus, and that he will be when he has gained an understanding of all the differences.
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.
Now, first, if the shape of the earth is not flat, its flatness cannot be the cause of its immobility.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.
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 centre 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 centre of the whirl.This is thought by all those who try to generate the heavens to explain why the earth came together at the centre.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.) But suppose both the 'whirl' and its flatness (the air beneath being withdrawn)