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 and cannot at any future time be truly said not to be.) If, again, the two terms are coincident, if the ungenerated is indestructible, and the indestructible ungenearted, then each of them is coincident with 'eternal'; anything ungenerated is eternal and anything indestructible is eternal.This is clear too from the definition of the terms, Whatever is destructible must be generated; for it is either ungenerated, or generated, but, if ungenerated, it is by hypothesis indestructible.Whatever, further, is generated must be destructible.For it is either destructible or indestructible, but, if indestructible, it is by hypothesis ungenerated.
If, however, 'indestructible' and 'ungenerated' are not coincident, there is no necessity that either the ungenerated or the indestructible should be eternal.But they must be coincident, for the following reasons.The terms 'generated' and 'destructible' are coincident; this is obvious from our former remarks, since between what always is and what always is not there is an intermediate which is neither, and that intermediate is the generated and destructible.
For whatever is either of these is capable both of being and of not being for a definite time: in either case, I mean, there is a certain period of time during which the thing is and another during which it is not.Anything therefore which is generated or destructible must be intermediate.Now let A be that which always is and B that which always is not, C the generated, and D the destructible.Then C
must be intermediate between A and B.For in their case there is no time in the direction of either limit, in which either A is not or B
is.But for the generated there must be such a time either actually or potentially, though not for A and B in either way.C then will be, and also not be, for a limited length of time, and this is true also of D, the destructible.Therefore each is both generated and destructible.
Therefore 'generated' and 'destructible' are coincident.Now let E
stand for the ungenerated, F for the generated, G for the indestructible, and H for the destructible.As for F and H, it has been shown that they are coincident.But when terms stand to one another as these do, F and H coincident, E and F never predicated of the same thing but one or other of everything, and G and H likewise, then E and G must needs be coincident.For suppose that E is not coincident with G, then F will be, since either E or F is predictable of everything.But of that of which F is predicated H will be predicable also.H will then be coincident with G, but this we saw to be impossible.And the same argument shows that G is coincident with E.