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On The Heavens   

bodies are manifestly endowed with weight and lightness, but an

assemblage of units can neither be composed to form a body nor possess



The necessity that each of the simple bodies should have a natural

movement may be shown as follows. They manifestly move, and if they

have no proper movement they must move by constraint: and the

constrained is the same as the unnatural. Now an unnatural movement

presupposes a natural movement which it contravenes, and which,

however many the unnatural movements, is always one. For naturally a

thing moves in one way, while its unnatural movements are manifold.

The same may be shown, from the fact of rest. Rest, also, must

either be constrained or natural, constrained in a place to which

movement was constrained, natural in a place movement to which was

natural. Now manifestly there is a body which is at rest at the

centre. If then this rest is natural to it, clearly motion to this

place is natural to it. If, on the other hand, its rest is

constrained, what is hindering its motion? Something, which is at

rest: but if so, we shall simply repeat the same argument; and

either we shall come to an ultimate something to which rest where it

is or we shall have an infinite process, which is impossible. The

hindrance to its movement, then, we will suppose, is a moving thing-as

Empedocles says that it is the vortex which keeps the earth still-:

but in that case we ask, where would it have moved to but for the

vortex? It could not move infinitely; for to traverse an infinite is

impossible, and impossibilities do not happen. So the moving thing

must stop somewhere, and there rest not by constraint but naturally.

But a natural rest proves a natural movement to the place of rest.

Hence Leucippus and Democritus, who say that the primary bodies are in

perpetual movement in the void or infinite, may be asked to explain

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