Incapacity is privation of capacity-i.e. of such a principle as
has been described either in general or in the case of something
that would naturally have the capacity, or even at the time when it
would naturally already have it; for the senses in which we should
call a boy and a man and a eunuch 'incapable of begetting' are
distinct.-Again, to either kind of capacity there is an opposite
incapacity-both to that which only can produce movement and to that
which can produce it well.
Some things, then, are called adunata in virtue of this kind of
incapacity, while others are so in another sense; i.e. both dunaton
and adunaton are used as follows. The impossible is that of which
the contrary is of necessity true, e.g. that the diagonal of a
square is commensurate with the side is impossible, because such a
statement is a falsity of which the contrary is not only true but also
necessary; that it is commensurate, then, is not only false but also
of necessity false. The contrary of this, the possible, is found
when it is not necessary that the contrary is false, e.g. that a man
should be seated is possible; for that he is not seated is not of
necessity false. The possible, then, in one sense, as has been said,
means that which is not of necessity false; in one, that which is
true; in one, that which may be true.-A 'potency' or 'power' in
geometry is so called by a change of meaning.-These senses of
'capable' or 'possible' involve no reference to potency. But the
senses which involve a reference to potency all refer to the primary
kind of potency; and this is a source of change in another thing or in
the same thing qua other.
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