Book 9
3
At the same time it is also clear that if B must be real if A is, then if it is possible for A to be real, it must also be possible for B to be real; for even if B is not necessarily possible, there is nothing to prevent its being possible. Let A, then, be possible. Then when A was possible, if A was assumed to be real, nothing impossible was involved; but B was necessarily real too. But ex hypothesi B was impossible. Let B be impossible.
4
Then if B is impossible, A must also be impossible. But A was by definition possible. Therefore so is B.
4
If, therefore, A is possible, B will also be possible; that is if their relation was such that if A is real, B must be real.
5
Then if, A and B being thus related, B is not possible on this condition, A and B will not be related as we assumed; and if when A is possible B is necessarily possible, then if A is real B must be real too. For to say that B must be possible if A is possible means that if A is real at the time when and in the way in which it was assumed that it was possible for it to be real, then B must be real at that time and in that way.
1
Since all potencies are either innate, like the senses, or acquired by practice, like flute-playing, or by study, as in the arts, some—such as are acquired by practice or a rational formula—we can only possess when we have first exercised themCf. Aristot. Met. 9.8.6, 7.; in the case of others which are not of this kind and which imply passivity, this is not necessary.
