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On The Heavens   

call simple bodies cannot be greater than it is. The motion of a

simple body must itself be simple, and we assert that there are only

these two simple motions, the circular and the straight, the latter

being subdivided into motion away from and motion towards the centre.


That there is no other form of motion opposed as contrary to the

circular may be proved in various ways. In the first place, there is

an obvious tendency to oppose the straight line to the circular. For

concave and convex are a not only regarded as opposed to one

another, but they are also coupled together and treated as a unity

in opposition to the straight. And so, if there is a contrary to

circular motion, motion in a straight line must be recognized as

having the best claim to that name. But the two forms of rectilinear

motion are opposed to one another by reason of their places; for up

and down is a difference and a contrary opposition in place. Secondly,

it may be thought that the same reasoning which holds good of the

rectilinear path applies also the circular, movement from A to B being

opposed as contrary to movement from B to A. But what is meant is

still rectilinear motion. For that is limited to a single path,

while the circular paths which pass through the same two points are

infinite in number. Even if we are confined to the single semicircle

and the opposition is between movement from C to D and from D to C

along that semicircle, the case is no better. For the motion is the

same as that along the diameter, since we invariably regard the

distance between two points as the length of the straight line which

joins them. It is no more satisfactory to construct a circle and treat

motion 'along one semicircle as contrary to motion along the other.

For example, taking a complete circle, motion from E to F on the

semicircle G may be opposed to motion from F to E on the semicircle H.

But even supposing these are contraries, it in no way follows that the

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