On The Heavens   

movement is unnatural. In that case, if it is the downward movement

which is unnatural, the upward movement will be natural; and if it

is the upward which is unnatural, the downward will be natural. For we

decided that of contrary movements, if the one is unnatural to

anything, the other will be natural to it. But since the natural

movement of the whole and of its part of earth, for instance, as a

whole and of a small clod-have one and the same direction, it results,

in the first place, that this body can possess no lightness or

heaviness at all (for that would mean that it could move by its own

nature either from or towards the centre, which, as we know, is

impossible); and, secondly, that it cannot possibly move in the way of

locomotion by being forced violently aside in an upward or downward

direction. For neither naturally nor unnaturally can it move with

any other motion but its own, either itself or any part of it, since

the reasoning which applies to the whole applies also to the part.

It is equally reasonable to assume that this body will be

ungenerated and indestructible and exempt from increase and

alteration, since everything that comes to be comes into being from

its contrary and in some substrate, and passes away likewise in a

substrate by the action of the contrary into the contrary, as we

explained in our opening discussions. Now the motions of contraries

are contrary. If then this body can have no contrary, because there

can be no contrary motion to the circular, nature seems justly to have

exempted from contraries the body which was to be ungenerated and

indestructible. For it is in contraries that generation and decay

subsist. Again, that which is subject to increase increases upon

contact with a kindred body, which is resolved into its matter. But

there is nothing out of which this body can have been generated. And

if it is exempt from increase and diminution, the same reasoning leads

us to suppose that it is also unalterable. For alteration is

movement in respect of quality; and qualitative states and

dispositions, such as health and disease, do not come into being