On The Heavens   

some say, the body so moved is fire, this movement is just as

unnatural to it as downward movement; for any one can see that fire

moves in a straight line away from the centre. On all these grounds,

therefore, we may infer with confidence that there is something beyond

the bodies that are about us on this earth, different and separate

from them; and that the superior glory of its nature is

proportionate to its distance from this world of ours.



3



In consequence of what has been said, in part by way of assumption

and in part by way of proof, it is clear that not every body either

possesses lightness or heaviness. As a preliminary we must explain

in what sense we are using the words 'heavy' and 'light',

sufficiently, at least, for our present purpose: we can examine the

terms more closely later, when we come to consider their essential

nature. Let us then apply the term 'heavy' to that which naturally

moves towards the centre, and 'light' to that which moves naturally

away from the centre. The heaviest thing will be that which sinks to

the bottom of all things that move downward, and the lightest that

which rises to the surface of everything that moves upward. Now,

necessarily, everything which moves either up or down possesses

lightness or heaviness or both-but not both relatively to the same

thing: for things are heavy and light relatively to one another;

air, for instance, is light relatively to water, and water light

relatively to earth. The body, then, which moves in a circle cannot

possibly possess either heaviness or lightness. For neither

naturally nor unnaturally can it move either towards or away from

the centre. Movement in a straight line certainly does not belong to

it naturally, since one sort of movement is, as we saw, appropriate to

each simple body, and so we should be compelled to identify it with

one of the bodies which move in this way. Suppose, then, that the