The Complete Aristotle (eng.)

changing,
some part must have changed (this is shown in the treatise on
movement), he who is learning must, it would seem, possess some
part of the science. But here too, then, it is clear that actuality
is in this sense also, viz. in order of generation and of time,
prior to potency.
    But (3) it is also prior in substantiality; firstly, (a) because
the things that are posterior in becoming are prior in form and in
substantiality (e.g. man is prior to boy and human being to seed;
for the one already has its form, and the other has not), and
because everything that comes to be moves towards a principle, i.e.
an end (for that for the sake of which a thing is, is its
principle, and the becoming is for the sake of the end), and the
actuality is the end, and it is for the sake of this that the
potency is acquired. For animals do not see in order that they may
have sight, but they have sight that they may see. And similarly
men have the art of building that they may build, and theoretical
science that they may theorize; but they do not theorize that they
may have theoretical science, except those who are learning by
practice; and these do not theorize except in a limited sense, or
because they have no need to theorize. Further, matter exists in a
potential state, just because it may come to its form; and when it
exists actually, then it is in its form. And the same holds good in
all cases, even those in which the end is a movement. And so, as
teachers think they have achieved their end when they have
exhibited the pupil at work, nature does likewise. For if this is
not the case, we shall have Pauson’s Hermes over again, since it
will be hard to say about the knowledge, as about the figure in the
picture, whether it is within or without. For the action is the
end, and the actuality is the action. And so even the word
‘actuality’ is derived from ‘action’, and points to the complete
reality.
    And while in some cases the exercise is the ultimate thing (e.g.
in sight the ultimate thing is seeing, and no other product besides
this results from sight), but from some things a product follows
(e.g. from the art of building there results a house as well as the
act of building), yet none the less the act is in the former case
the end and in the latter more of an end than the potency is. For
the act of building is realized in the thing that is being built,
and comes to be, and is, at the same time as the house.
    Where, then, the result is something apart from the exercise,
the actuality is in the thing that is being made, e.g. the act of
building is in the thing that is being built and that of weaving in
the thing that is being woven, and similarly in all other cases,
and in general the movement is in the thing that is being moved;
but where there is no product apart from the actuality, the
actuality is present in the agents, e.g. the act of seeing is in
the seeing subject and that of theorizing in the theorizing subject
and the life is in the soul (and therefore well-being also; for it
is a certain kind of life).
    Obviously, therefore, the substance or form is actuality.
According to this argument, then, it is obvious that actuality is
prior in substantial being to potency; and as we have said, one
actuality always precedes another in time right back to the
actuality of the eternal prime mover.
    But (b) actuality is prior in a stricter sense also; for eternal
things are prior in substance to perishable things, and no eternal
thing exists potentially. The reason is this. Every potency is at
one and the same time a potency of the opposite; for, while that
which is not capable of being present in a subject cannot be
present, everything that is capable of being may possibly not be
actual. That, then, which is capable of being may either be or not
be; the same thing, then, is capable both of being and of not
being. And that which is capable of not being may possibly not be;
and that which may possibly not be is perishable, either in the
full sense, or in the precise sense in which it is said that it
possibly may not be, i.e. in respect either of place or of quantity
or quality; ‘in the full sense’ means ‘in respect of substance’.
Nothing, then, which is in the full sense imperishable is in the
full sense potentially existent (though there is nothing to prevent
its being so in some respect, e.g. potentially of a certain quality
or in a certain place); all imperishable things, then, exist
actually. Nor can anything which