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On The Heavens   

well be an irregular movement in the moved. But none of these

possibilities can be conceived as actual in the case of the heavens.

As to that which is moved, we have shown that it is primary and simple

and ungenerated and indestructible and generally unchanging; and the

mover has an even better right to these attributes. It is the

primary that moves the primary, the simple the simple, the

indestructible and ungenerated that which is indestructible and

ungenerated. Since then that which is moved, being a body, is

nevertheless unchanging, how should the mover, which is incorporeal,

be changed?

It follows then, further, that the motion cannot be irregular. For

if irregularity occurs, there must be change either in the movement as

a whole, from fast to slow and slow to fast, or in its parts. That

there is no irregularity in the parts is obvious, since, if there

were, some divergence of the stars would have taken place before now

in the infinity of time, as one moved slower and another faster: but

no alteration of their intervals is ever observed. Nor again is a

change in the movement as a whole admissible. Retardation is always

due to incapacity, and incapacity is unnatural. The incapacities of

animals, age, decay, and the like, are all unnatural, due, it seems,

to the fact that the whole animal complex is made up of materials

which differ in respect of their proper places, and no single part

occupies its own place. If therefore that which is primary contains

nothing unnatural, being simple and unmixed and in its proper place

and having no contrary, then it has no place for incapacity, nor,

consequently, for retardation or (since acceleration involves

retardation) for acceleration. Again, it is inconceivable that the

mover should first show incapacity for an infinite time, and

capacity afterwards for another infinity. For clearly nothing which,

like incapacity, unnatural ever continues for an infinity of time; nor

does the unnatural endure as long as the natural, or any form of

incapacity as long as the capacity. But if the movement is retarded it

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