One might raise the question whether “possible to be” follows from “necessary to be”. For if not, the contradictory will follow, “not possible to be”. And even if one were to state that this is not the contradictory, one must say that “possible not to be” is; both of which are false of “necessary to be”. However, the same thing seems to be capable of being cut and of not being cut, of being and of not being, so that the necessary will be contingent not to be.
It is clear then that not everything capable either of being or of walking is capable of the opposites also, but there are things of which this is not true. Firstly, with things which are not capable rationally; fire, for instance, is capable of heating and has an irrational capability. So, while the rational capabilities are the same for more and opposites, the irrational capabilities are not always thus, but fire, as has been said, is not capable of heating and of not heating, nor everything else that is always actualized. Yet some even of the things with irrational capabilities are at the same time capable of opposites; for example, the cloak is equally capable of being cut up and of not being cut up. But our point here is that not every capability is for opposites, not even those which are said of the same kind (i.e. the irrational capabilities). Again, some capabilities are homonymous. For the capable is not spoken of unconditionally, but in one way it reflects that it is true as being actualized (e.g. it is capable of walking because it walks, and in general capable of being because what is called capable already exists in actuality), in another that it might be actualized (e.g. it is capable of walking because it might walk).
This latter capability holds of changeable things only (as changeable things we call those which are capable of changing by nature – not, however, that they have already changed; such things are only those belonging to the world of generation and destruction); the former to unchangeable things also (for even in changeable things this kind of capability is once considered, when they receive their own accomplishment, to which they had led their potentially capability; but before them in the constantly changeable things, just as with haven bodies – yet having their potentially capability combined with actuality; previously, in the unchangeable things, that is, the intelligible and divine causes, whereat there is no place for the potentially capability). Of both it is true to say that it is not possible for them to walk or to be, both what is already walking and actualized and what is capable of walking.
So it is not true to predicate the latter kind of capability (the potentially capable) of that which is unconditionally necessary, but it is true for the former (the actually capable). Consequently, since the universal follows from the particular, from that which is necessary there follows its capability of being, though not in every case of capable. Perhaps, indeed, the necessary and not necessary are principles of everything either of being or of not being, and the others must be considered as following from these.
It is plain from what has been said that what is of necessity is in actuality; so, if those which are eternal are prior, actuality also is prior to capability. Some things are actualities without capability, e.g. the primary substances, others with capability, e.g. in those things that are brought about, which are prior by nature but posterior in time (of these capability will be prior to actuality in time but posterior in nature; for first one must have been potentially capable of a certain “this”, so that later on to be led to that which it had been thus.
But since actuality is something perfect while capability something imperfect – and perfect is prior in nature to imperfect – it is clear that what is in actuality will be prior to what is in potentiality. For if the man in actuality did not exist before, how could the fetus, which we call “potentially man”, be?); and others are never actualities but only capabilities, such as the infinite division of the continuous quantities and the number’s increase. For none of these could ever be infinitude in actuality either of the division or of the increase, and they have the potentiality of this only: of being capable indefinitely.
Bibliography: Aristotle De Interpretatione, Ammonius in Aristotle’s De Interpretatione, J.L. Ackrill in Aristotle’s Categories and De Interpretatione
Translation – text editing: George Kotsalis