The Complete Aristotle (eng.)

impossible. Let it be impossible then. If, then, B
is impossible, A also must be so. But the first was supposed
impossible; therefore the second also is impossible. If, then, A is
possible, B also will be possible, if they were so related that if
A,is real, B must be real. If, then, A and B being thus related, B
is not possible on this condition, and B will not be related as was
supposed. And if when A is possible, B must be possible, then if A
is real, B also must be real. For to say that B must be possible,
if A is possible, means this, that if A is real both at the time
when and in the way in which it was supposed capable of being real,
B also must then and in that way be real.
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5
    As all potencies are either innate, like the senses, or come by
practice, like the power of playing the flute, or by learning, like
artistic power, those which come by practice or by rational formula
we must acquire by previous exercise but this is not necessary with
those which are not of this nature and which imply passivity.
    Since that which is ‘capable’ is capable of something and at
some time in some way (with all the other qualifications which must
be present in the definition), and since some things can produce
change according to a rational formula and their potencies involve
such a formula, while other things are nonrational and their
potencies are non-rational, and the former potencies must be in a
living thing, while the latter can be both in the living and in the
lifeless; as regards potencies of the latter kind, when the agent
and the patient meet in the way appropriate to the potency in
question, the one must act and the other be acted on, but with the
former kind of potency this is not necessary. For the nonrational
potencies are all productive of one effect each, but the rational
produce contrary effects, so that if they produced their effects
necessarily they would produce contrary effects at the same time;
but this is impossible. There must, then, be something else that
decides; I mean by this, desire or will. For whichever of two
things the animal desires decisively, it will do, when it is
present, and meets the passive object, in the way appropriate to
the potency in question. Therefore everything which has a rational
potency, when it desires that for which it has a potency and in the
circumstances in which it has the potency, must do this. And it has
the potency in question when the passive object is present and is
in a certain state; if not it will not be able to act. (To add the
qualification ‘if nothing external prevents it’ is not further
necessary; for it has the potency on the terms on which this is a
potency of acting, and it is this not in all circumstances but on
certain conditions, among which will be the exclusion of external
hindrances; for these are barred by some of the positive
qualifications.) And so even if one has a rational wish, or an
appetite, to do two things or contrary things at the same time, one
will not do them; for it is not on these terms that one has the
potency for them, nor is it a potency of doing both at the same
time, since one will do the things which it is a potency of doing,
on the terms on which one has the potency.
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6
    Since we have treated of the kind of potency which is related to
movement, let us discuss actuality-what, and what kind of thing,
actuality is. For in the course of our analysis it will also become
clear, with regard to the potential, that we not only ascribe
potency to that whose nature it is to move something else, or to be
moved by something else, either without qualification or in some
particular way, but also use the word in another sense, which is
the reason of the inquiry in the course of which we have discussed
these previous senses also. Actuality, then, is the existence of a
thing not in the way which we express by ‘potentially’; we say that
potentially, for instance, a statue of Hermes is in the block of
wood and the half-line is in the whole, because it might be
separated out, and we call even the man who is not studying a man
of science, if he is capable of studying; the thing that stands in
contrast to each of these exists actually. Our meaning can be seen
in the particular cases by induction, and we must not seek a
definition of everything but be content to grasp the analogy, that
it is as that which is building is to that which is