Having established these distinctions we car now proceed to the sequel.If there are thing! capable both of being and of not being, there must be some definite maximum time of their being and not being;
a time, I mean, during which continued existence is possible to them and a time during which continued nonexistence is possible.And this is true in every category, whether the thing is, for example, 'man', or 'white', or 'three cubits long', or whatever it may be.For if the time is not definite in quantity, but longer than any that can be suggested and shorter than none, then it will be possible for one and the same thing to exist for infinite time and not to exist for another infinity.This, however, is impossible.
Let us take our start from this point.The impossible and the false have not the same significance.One use of 'impossible' and 'possible', and 'false' and 'true', is hypothetical.It is impossible, for instance, on a certain hypothesis that the triangle should have its angles equal to two right angles, and on another the diagonal is commensurable.But there are also things possible and impossible, false and true, absolutely.Now it is one thing to be absolutely false, and another thing to be absolutely impossible.To say that you are standing when you are not standing is to assert a falsehood, but not an impossibility.Similarly to say that a man who is playing the harp, but not singing, is singing, is to say what is false but not impossible.To say, however, that you are at once standing and sitting, or that the diagonal is commensurable, is to say what is 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 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.
