On The Heavens   

away from the centre, and 'downward' motion towards it. All simple

motion, then, must be motion either away from or towards or about

the centre. This seems to be in exact accord with what we said

above: as body found its completion in three dimensions, so its

movement completes itself in three forms.

Bodies are either simple or compounded of such; and by simple bodies

I mean those which possess a principle of movement in their own

nature, such as fire and earth with their kinds, and whatever is

akin to them. Necessarily, then, movements also will be either

simple or in some sort compound-simple in the case of the simple

bodies, compound in that of the composite-and in the latter case the

motion will be that of the simple body which prevails in the

composition. Supposing, then, that there is such a thing as simple

movement, and that circular movement is an instance of it, and that

both movement of a simple body is simple and simple movement is of a

simple body (for if it is movement of a compound it will be in

virtue of a prevailing simple element), then there must necessarily be

some simple body which revolves naturally and in virtue of its own

nature with a circular movement. By constraint, of course, it may be

brought to move with the motion of something else different from

itself, but it cannot so move naturally, since there is one sort of

movement natural to each of the simple bodies. Again, if the unnatural

movement is the contrary of the natural and a thing can have no more

than one contrary, it will follow that circular movement, being a

simple motion, must be unnatural, if it is not natural, to the body

moved. If then (1) the body, whose movement is circular, is fire or

some other element, its natural motion must be the contrary of the

circular motion. But a single thing has a single contrary; and

upward and downward motion are the contraries of one another. If, on

the other hand, (2) the body moving with this circular motion which is

unnatural to it is something different from the elements, there will

be some other motion which is natural to it. But this cannot be. For