The Complete Aristotle (eng.)

so with the other arts. If, then, it is impossible to have such
arts if one has not at some time learnt and acquired them, and it
is then impossible not to have them if one has not sometime lost
them (either by forgetfulness or by some accident or by time; for
it cannot be by the destruction of the object, for that lasts for
ever), a man will not have the art when he has ceased to use it,
and yet he may immediately build again; how then will he have got
the art? And similarly with regard to lifeless things; nothing will
be either cold or hot or sweet or perceptible at all if people are
not perceiving it; so that the upholders of this view will have to
maintain the doctrine of Protagoras. But, indeed, nothing will even
have perception if it is not perceiving, i.e. exercising its
perception. If, then, that is blind which has not sight though it
would naturally have it, when it would naturally have it and when
it still exists, the same people will be blind many times in the
day-and deaf too.
    Again, if that which is deprived of potency is incapable, that
which is not happening will be incapable of happening; but he who
says of that which is incapable of happening either that it is or
that it will be will say what is untrue; for this is what
incapacity meant. Therefore these views do away with both movement
and becoming. For that which stands will always stand, and that
which sits will always sit, since if it is sitting it will not get
up; for that which, as we are told, cannot get up will be incapable
of getting up. But we cannot say this, so that evidently potency
and actuality are different (but these views make potency and
actuality the same, and so it is no small thing they are seeking to
annihilate), so that it is possible that a thing may be capable of
being and not he, and capable of not being and yet he, and
similarly with the other kinds of predicate; it may be capable of
walking and yet not walk, or capable of not walking and yet walk.
And a thing is capable of doing something if there will be nothing
impossible in its having the actuality of that of which it is said
to have the capacity. I mean, for instance, if a thing is capable
of sitting and it is open to it to sit, there will be nothing
impossible in its actually sitting; and similarly if it is capable
of being moved or moving, or of standing or making to stand, or of
being or coming to be, or of not being or not coming to be.
    The word ‘actuality’, which we connect with ‘complete reality’,
has, in the main, been extended from movements to other things; for
actuality in the strict sense is thought to be identical with
movement. And so people do not assign movement to non-existent
things, though they do assign some other predicates. E.g. they say
that non-existent things are objects of thought and desire, but not
that they are moved; and this because, while ex hypothesi they do
not actually exist, they would have to exist actually if they were
moved. For of non-existent things some exist potentially; but they
do not exist, because they do not exist in complete reality.
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4
    If what we have described is identical with the capable or
convertible with it, evidently it cannot be true to say ‘this is
capable of being but will not be’, which would imply that the
things incapable of being would on this showing vanish. Suppose,
for instance, that a man-one who did not take account of that which
is incapable of being-were to say that the diagonal of the square
is capable of being measured but will not be measured, because a
thing may well be capable of being or coming to be, and yet not be
or be about to be. But from the premisses this necessarily follows,
that if we actually supposed that which is not, but is capable of
being, to be or to have come to be, there will be nothing
impossible in this; but the result will be impossible, for the
measuring of the diagonal is impossible. For the false and the
impossible are not the same; that you are standing now is false,
but that you should be standing is not impossible.
    At the same time it is clear that if, when A is real, B must be
real, then, when A is possible, B also must be possible. For if B
need not be possible, there is nothing to prevent its not being
possible. Now let A be supposed possible. Then, when A was
possible, we agreed that nothing impossible followed if A were
supposed to be real; and then B must of course be real. But we
supposed B to be