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On The Heavens   



possible, and, just as upward movement is the superior form of

rectilinear movement, since the upper region is more divine than the

lower, so forward movement is superior to backward, then front and

back exhibits, like right and left, as we said before and as the

difficulty just stated itself suggests, the distinction of prior and

posterior, which provides a reason and so solves our difficulty.

Supposing that nature is ordered in the best way possible, this may

stand as the reason of the fact mentioned. For it is best to move with

a movement simple and unceasing, and, further, in the superior of

two possible directions.



6



We have next to show that the movement of the heaven is regular

and not irregular. This applies only to the first heaven and the first

movement; for the lower spheres exhibit a composition of several

movements into one. If the movement is uneven, clearly there will be

acceleration, maximum speed, and retardation, since these appear in

all irregular motions. The maximum may occur either at the

starting-point or at the goal or between the two; and we expect

natural motion to reach its maximum at the goal, unnatural motion at

the starting-point, and missiles midway between the two. But

circular movement, having no beginning or limit or middle in the

direct sense of the words, has neither whence nor whither nor

middle: for in time it is eternal, and in length it returns upon

itself without a break. If then its movement has no maximum, it can

have no irregularity, since irregularity is produced by retardation

and acceleration. Further, since everything that is moved is moved

by something, the cause of the irregularity of movement must lie

either in the mover or in the moved or both. For if the mover moved

not always with the same force, or if the moved were altered and did

not remain the same, or if both were to change, the result might

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