On The Heavens   

not only false but also impossible. Thus it is not the same thing to

make a false and to make an impossible hypothesis, and from the

impossible hypothesis impossible results follow. A man has, it is

true, the capacity at once of sitting and of standing, because when he

possesses the one he also possesses the other; but it does not

follow that he can at once sit and stand, only that at another time he

can do the other also. But if a thing has for infinite time more

than one capacity, another time is impossible and the times must

coincide. Thus if a thing which exists for infinite time is

destructible, it will have the capacity of not being. Now if it exists

for infinite time let this capacity be actualized; and it will be in

actuality at once existent and non-existent. Thus a false conclusion

would follow because a false assumption was made, but if what was

assumed had not been impossible its consequence would not have been

impossible.

Anything then which always exists is absolutely imperishable. It

is also ungenerated, since if it was generated it will have the

power for some time of not being. For as that which formerly was,

but now is not, or is capable at some future time of not being, is

destructible, so that which is capable of formerly not having been

is generated. But in the case of that which always is, there is no

time for such a capacity of not being, whether the supposed time is

finite or infinite; for its capacity of being must include the

finite time since it covers infinite time.

It is therefore impossible that one and the same thing should be

capable of always existing and of always not-existing. And 'not always

existing', the contradictory, is also excluded. Thus it is

impossible for a thing always to exist and yet to be destructible.

Nor, similarly, can it be generated. For of two attributes if B cannot

be present without A, the impossibility A of proves the

impossibility of B. What always is, then, since it is incapable of

ever not being, cannot possibly be generated. But since the