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Works by Aristotle
Pages of Metaphysics

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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.

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 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

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