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 the manner of their motion and the kind of movement which is natural to them.For if the various elements are constrained by one another to move as they do, each must still have a natural movement which the constrained contravenes, and the prime mover must cause motion not by constraint but naturally.If there is no ultimate natural cause of movement and each preceding term in the series is always moved by constraint, we shall have an infinite process.The same difficulty is involved even if it is supposed, as we read in the Timaeus, that before the ordered world was made the elements moved without order.
Their movement must have been due either to constraint or to their nature.And if their movement was natural, a moment's consideration shows that there was already an ordered world.For the prime mover must cause motion in virtue of its own natural movement, and the other bodies, moving without constraint, as they came to rest in their proper places, would fall into the order in which they now stand, the heavy bodies moving towards the centre and the light bodies away from it.But that is the order of their distribution in our world.
