第36章

cease to prevent the earth's motion, where will the earth move to then? Its movement to the centre was constrained, and its rest at the centre 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.

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.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 centre, the question remains, why now do all heavy bodies move to the earth.For the whirl surely does not come near us.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.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: but there are some, Anaximander, for instance, among the ancients, who say that the earth keeps its place because of its indifference.Motion upward and downward and sideways were all, they thought, equally inappropriate to that which is set at the centre and indifferently related to every extreme point; and to move in contrary directions at the same time was impossible: so it must needs remain still.This view is ingenious but not true.The argument would prove that everything, whatever it be, which is put at the centre, must stay there.Fire, then, will rest at the centre: for the proof turns on no peculiar property of earth.But this does not follow.The observed facts about earth are not only that it remains at the centre, but also that it moves to the centre.The place to which any fragment of earth moves must necessarily be the place to which the whole moves; and in the place to which a thing naturally moves, it will naturally rest.The reason then is not in the fact that the earth is indifferently related to every extreme point: for this would apply to any body, whereas movement to the centre is peculiar to earth.Again it is absurd to look for a reason why the earth remains at the centre and not for a reason why fire remains at the extremity.If the extremity is the natural place of fire, clearly earth must also have a natural place.But suppose that the centre is not its place, and that the reason of its remaining there is this necessity of indifference-on the analogy of the hair which, it is said, however great the tension, will not break under it, if it be evenly distributed, or of the men who, though exceedingly hungry and thirsty, and both equally, yet being equidistant from food and drink, is therefore bound to stay where he is-even so, it still remains to explain why fire stays at the extremities.It is strange, too, to ask about things staying still but not about their motion,-why, I mean, one thing, if nothing stops it, moves up, and another thing to the centre.Again, their statements are not true.

It happens, indeed, to be the case that a thing to which movement this way and that is equally inappropriate is obliged to remain at the centre.But so far as their argument goes, instead of remaining there, it will move, only not as a mass but in fragments.For the argument applies equally to fire.Fire, if set at the centre, should stay there, like earth, since it will be indifferently related to every point on the extremity.Nevertheless it will move, as in fact it always does move when nothing stops it, away from the centre to the extremity.It will not, however, move in a mass to a single point on the circumference-the only possible result on the lines of the indifference theory-but rather each corresponding portion of fire to the corresponding part of the extremity, each fourth part, for instance, to a fourth part of the circumference.For since no body is a point, it will have parts.The expansion, when the body increased the place occupied, would be on the same principle as the contraction, in which the place was diminished.Thus, for all the indifference theory shows to the contrary, earth also would have moved in this manner away from the centre, unless the centre had been its natural place.

We have now outlined the views held as to the shape, position, and rest or movement of the earth.