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On The Heavens   



must necessarily be retarded for an infinite time. Equally

impossible is perpetual acceleration or perpetual retardation. For

such movement would be infinite and indefinite, but every movement, in

our view, proceeds from one point to another and is definite in

character. Again, suppose one assumes a minimum time in less than

which the heaven could not complete its movement. For, as a given walk

or a given exercise on the harp cannot take any and every time, but

every performance has its definite minimum time which is

unsurpassable, so, one might suppose, the movement of the heaven could

not be completed in any and every time. But in that case perpetual

acceleration is impossible (and, equally, perpetual retardation: for

the argument holds of both and each), if we may take acceleration to

proceed by identical or increasing additions of speed and for an

infinite time. The remaining alternative is to say that the movement

exhibits an alternation of slower and faster: but this is a mere

fiction and quite inconceivable. Further, irregularity of this kind

would be particularly unlikely to pass unobserved, since contrast

makes observation easy.

That there is one heaven, then, only, and that it is ungenerated and

eternal, and further that its movement is regular, has now been

sufficiently explained.



7



We have next to speak of the stars, as they are called, of their

composition, shape, and movements. It would be most natural and

consequent upon what has been said that each of the stars should be

composed of that substance in which their path lies, since, as we

said, there is an element whose natural movement is circular. In so

saying we are only following the same line of thought as those who say

that the stars are fiery because they believe the upper body to be

fire, the presumption being that a thing is composed of the same stuff

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