On The Heavens   

contradictory of 'that which is always capable of being' 'that which

is not always capable of being'; while 'that which is always capable

of not being' is the contrary, whose contradictory in turn is 'that

which is not always capable of not being', it is necessary that the

contradictories of both terms should be predicable of one and the same

thing, and thus that, intermediate between what always is and what

always is not, there should be that to which being and not-being are

both possible; for the contradictory of each will at times be true

of it unless it always exists. Hence that which not always is not will

sometimes be and sometimes not be; and it is clear that this is true

also of that which cannot always be but sometimes is and therefore

sometimes is not. One thing, then, will have the power of being, and

will thus be intermediate between the other two.

Expresed universally our argument is as follows. Let there be two

attributes, A and B, not capable of being present in any one thing

together, while either A or C and either B or D are capable of being

present in everything. Then C and D must be predicated of everything

of which neither A nor B is predicated. Let E lie between A and B; for

that which is neither of two contraries is a mean between them. In E

both C and D must be present, for either A or C is present

everywhere and therefore in E. Since then A is impossible, C must be

present, and the same argument holds of D.

Neither that which always is, therefore, nor that which always is

not is either generated or destructible. And clearly whatever is

generated or destructible is not eternal. If it were, it would be at

once capable of always being and capable of not always being, but it

has already been shown that this is impossible. Surely then whatever

is ungenerated and in being must be eternal, and whatever is

indestructible and in being must equally be so. (I use the words

'ungenerated' and 'indestructible' in their proper sense,

'ungenerated' for that which now is and could not at any previous time

have been truly said not to be; 'indestructible' for that which now is