On The Heavens   

The word 'ungenerated' then is used (a) in one sense whenever

something now is which formerly was not, no process of becoming or

change being involved. Such is the case, according to some, with

contact and motion, since there is no process of coming to be in

contact or in motion. (b) It is used in another sense, when

something which is capable of coming to be, with or without process,

does not exist; such a thing is ungenerated in the sense that its

generation is not a fact but a possibility. (c) It is also applied

where there is general impossibility of any generation such that the

thing now is which then was not. And 'impossibility' has two uses:

first, where it is untrue to say that the thing can ever come into

being, and secondly, where it cannot do so easily, quickly, or well.

In the same way the word 'generated' is used, (a) first, where what

formerly was not afterwards is, whether a process of becoming was or

was not involved, so long as that which then was not, now is; (b)

secondly, of anything capable of existing, 'capable' being defined

with reference either to truth or to facility; (c) thirdly, of

anything to which the passage from not being to being belongs, whether

already actual, if its existence is due to a past process of becoming,

or not yet actual but only possible. The uses of the words

'destructible' and 'indestructible' are similar. 'Destructible' is

applied (a) to that which formerly was and afterwards either is not or

might not be, whether a period of being destroyed and changed

intervenes or not; and (b) sometimes we apply the word to that which a

process of destruction may cause not to be; and also (c) in a third

sense, to that which is easily destructible, to the 'easily

destroyed', so to speak. Of the indestructible the same account

holds good. It is either (a) that which now is and now is not, without

any process of destruction, like contact, which without being

destroyed afterwards is not, though formerly it was; or (b) that which

is but might not be, or which will at some time not be, though it

now is. For you exist now and so does the contact; yet both are