On The Heavens   

destructible, because a time will come when it will not be true of you

that you exist, nor of these things that they are in contact.

Thirdly (c) in its most proper use, it is that which is, but is

incapable of any destruction such that the thing which now is later

ceases to be or might cease to be; or again, that which has not yet

been destroyed, but in the future may cease to be. For

indestructible is also used of that which is destroyed with

difficulty.

This being so, we must ask what we mean by 'possible' and

'impossible'. For in its most proper use the predicate

'indestructible' is given because it is impossible that the thing

should be destroyed, i.e. exist at one time and not at another. And

'ungenerated' also involves impossibility when used for that which

cannot be generated, in such fashion that, while formerly it was

not, later it is. An instance is a commensurable diagonal. Now when we

speak of a power to move or to lift weights, we refer always to the

maximum. We speak, for instance, of a power to lift a hundred

talents or walk a hundred stades-though a power to effect the

maximum is also a power to effect any part of the maximum-since we

feel obliged in defining the power to give the limit or maximum. A

thing, then, which is within it. If, for example, a man can lift a

hundred talents, he can also lift two, and if he can walk a hundred

stades, he can also walk two. But the power is of the maximum, and a

thing said, with reference to its maximum, to be incapable of so

much is also incapable of any greater amount. It is, for instance,

clear that a person who cannot walk a thousand stades will also be

unable to walk a thousand and one. This point need not trouble us, for

we may take it as settled that what is, in the strict sense,

possible is determined by a limiting maximum. Now perhaps the

objection might be raised that there is no necessity in this, since he

who sees a stade need not see the smaller measures contained in it,

while, on the contrary, he who can see a dot or hear a small sound