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On The Heavens   

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both the 'whirl' and its flatness (the air beneath being withdrawn)

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

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