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On The Heavens   

generation either of everything or in an absolute sense of anything.

It is impossible that everything should be generated, unless an

extra-corporeal void is possible. For, assuming generation, the

place which is to be occupied by that which is coming to be, must have

been previously occupied by void in which no body was. Now it is quite

possible for one body to be generated out of another, air for instance

out of fire, but in the absence of any pre-existing mass generation is

impossible. That which is potentially a certain kind of body may, it

is true, become such in actuality, But if the potential body was not

already in actuality some other kind of body, the existence of an

extra-corporeal void must be admitted.


It remains to say what bodies are subject to generation, and why.

Since in every case knowledge depends on what is primary, and the

elements are the primary constituents of bodies, we must ask which

of such bodies are elements, and why; and after that what is their

number and character. The answer will be plain if we first explain

what kind of substance an element is. An element, we take it, is a

body into which other bodies may be analysed, present in them

potentially or in actuality (which of these, is still disputable), and

not itself divisible into bodies different in form. That, or something

like it, is what all men in every case mean by element. Now if what we

have described is an element, clearly there must be such bodies. For

flesh and wood and all other similar bodies contain potentially fire

and earth, since one sees these elements exuded from them; and, on the

other hand, neither in potentiality nor in actuality does fire contain

flesh or wood, or it would exude them. Similarly, even if there were

only one elementary body, it would not contain them. For though it

will be either flesh or bone or something else, that does not at

once show that it contained these in potentiality: the further

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