Now the relation of the ungenerated (E) to the generated (F) is the same as that of the indestructible (G) to the destructible (H).To say then that there is no reason why anything should not be generated and yet indestructible or ungenerated and yet destroyed, to imagine that in the one case generation and in the other case destruction occurs once for all, is to destroy part of the data.For (1) everything is capable of acting or being acted upon, of being or not being, either for an infinite, or for a definitely limited space of time; and the infinite time is only a possible alternative because it is after a fashion defined, as a length of time which cannot be exceeded.But infinity in one direction is neither infinite or finite.(2) Further, why, after always existing, was the thing destroyed, why, after an infinity of not being, was it generated, at one moment rather than another? If every moment is alike and the moments are infinite in number, it is clear that a generated or destructible thing existed for an infinite time.It has therefore for an infinite time the capacity of not being (since the capacity of being and the capacity of not being will be present together), if destructible, in the time before destruction, if generated, in the time after generation.If then we assume the two capacities to be actualized, opposites will be present together.(3) Further, this second capacity will be present like the first at every moment, so that the thing will have for an infinite time the capacity both of being and of not being; but this has been shown to be impossible.
(4) Again, if the capacity is present prior to the activity, it will be present for all time, even while the thing was as yet ungenerated and non-existent, throughout the infinite time in which it was capable of being generated.At that time, then, when it was not, at that same time it had the capacity of being, both of being then and of being thereafter, and therefore for an infinity of time.
It is clear also on other grounds that it is impossible that the destructible should not at some time be destroyed.For otherwise it will always be at once destructible and in actuality indestructible, so that it will be at the same time capable of always existing and of not always existing.Thus the destructible is at some time actually destroyed.The generable, similarly, has been generated, for it is capable of having been generated and thus also of not always existing.
We may also see in the following way how impossible it is either for a thing which is generated to be thenceforward indestructible, or for a thing which is ungenerated and has always hitherto existed to be destroyed.Nothing that is by chance can be indestructible or ungenerated, since the products of chance and fortune are opposed to what is, or comes to be, always or usually, while anything which exists for a time infinite either absolutely or in one direction, is in existence either always or usually.That which is by chance, then, is by nature such as to exist at one time and not at another.
But in things of that character the contradictory states proceed from one and the same capacity, the matter of the thing being the cause equally of its existence and of its non-existence.Hence contradictories would be present together in actuality.
Further, it cannot truly be said of a thing now that it exists last year, nor could it be said last year that it exists now.It is therefore impossible for what once did not exist later to be eternal.For in its later state it will possess the capacity of not existing, only not of not existing at a time when it exists-since then it exists in actuality-but of not existing last year or in the past.
Now suppose it to be in actuality what it is capable of being.It will then be true to say now that it does not exist last year.But this is impossible.No capacity relates to being in the past, but always to being in the present or future.It is the same with the notion of an eternity of existence followed later by non-existence.In the later state the capacity will be present for that which is not there in actuality.Actualize, then, the capacity.It will be true to say now that this exists last year or in the past generally.
Considerations also not general like these but proper to the subject show it to be impossible that what was formerly eternal should later be destroyed or that what formerly was not should later be eternal.
Whatever is destructible or generated is always alterable.Now alteration is due to contraries, and the things which compose the natural body are the very same that destroy it.