On The Heavens   

being and of not being; but this has been shown to be impossible.

(4) Again, if the capacity is present prior to the activity, it will

be present for all time, even while the thing was as yet ungenerated

and non-existent, throughout the infinite time in which it was capable

of being generated. At that time, then, when it was not, at that

same time it had the capacity of being, both of being then and of

being thereafter, and therefore for an infinity of time.

It is clear also on other grounds that it is impossible that the

destructible should not at some time be destroyed. For otherwise it

will always be at once destructible and in actuality indestructible,

so that it will be at the same time capable of always existing and

of not always existing. Thus the destructible is at some time actually

destroyed. The generable, similarly, has been generated, for it is

capable of having been generated and thus also of not always existing.

We may also see in the following way how impossible it is either for

a thing which is generated to be thenceforward indestructible, or

for a thing which is ungenerated and has always hitherto existed to be

destroyed. Nothing that is by chance can be indestructible or

ungenerated, since the products of chance and fortune are opposed to

what is, or comes to be, always or usually, while anything which

exists for a time infinite either absolutely or in one direction, is

in existence either always or usually. That which is by chance,

then, is by nature such as to exist at one time and not at another.

But in things of that character the contradictory states proceed

from one and the same capacity, the matter of the thing being the

cause equally of its existence and of its non-existence. Hence

contradictories would be present together in actuality.

Further, it cannot truly be said of a thing now that it exists

last year, nor could it be said last year that it exists now. It is

therefore impossible for what once did not exist later to be

eternal. For in its later state it will possess the capacity of not

existing, only not of not existing at a time when it exists-since then