The Complete Aristotle (eng.)

can
itself be acted on or because something else can be acted on by
it), but in a sense the potencies are different. For the one is in
the thing acted on; it is because it contains a certain originative
source, and because even the matter is an originative source, that
the thing acted on is acted on, and one thing by one, another by
another; for that which is oily can be burnt, and that which yields
in a particular way can be crushed; and similarly in all other
cases. But the other potency is in the agent, e.g. heat and the art
of building are present, one in that which can produce heat and the
other in the man who can build. And so, in so far as a thing is an
organic unity, it cannot be acted on by itself; for it is one and
not two different things. And ‘impotence’and ‘impotent’ stand for
the privation which is contrary to potency of this sort, so that
every potency belongs to the same subject and refers to the same
process as a corresponding impotence. Privation has several senses;
for it means (1) that which has not a certain quality and (2) that
which might naturally have it but has not it, either (a) in general
or (b) when it might naturally have it, and either (a) in some
particular way, e.g. when it has not it completely, or (b) when it
has not it at all. And in certain cases if things which naturally
have a quality lose it by violence, we say they have suffered
privation.
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2
    Since some such originative sources are present in soulless
things, and others in things possessed of soul, and in soul, and in
the rational part of the soul, clearly some potencies will, be
non-rational and some will be non-rational and some will be
accompanied by a rational formula. This is why all arts, i.e. all
productive forms of knowledge, are potencies; they are originative
sources of change in another thing or in the artist himself
considered as other.
    And each of those which are accompanied by a rational formula is
alike capable of contrary effects, but one non-rational power
produces one effect; e.g. the hot is capable only of heating, but
the medical art can produce both disease and health. The reason is
that science is a rational formula, and the same rational formula
explains a thing and its privation, only not in the same way; and
in a sense it applies to both, but in a sense it applies rather to
the positive fact. Therefore such sciences must deal with
contraries, but with one in virtue of their own nature and with the
other not in virtue of their nature; for the rational formula
applies to one object in virtue of that object’s nature, and to the
other, in a sense, accidentally. For it is by denial and removal
that it exhibits the contrary; for the contrary is the primary
privation, and this is the removal of the positive term. Now since
contraries do not occur in the same thing, but science is a potency
which depends on the possession of a rational formula, and the soul
possesses an originative source of movement; therefore, while the
wholesome produces only health and the calorific only heat and the
frigorific only cold, the scientific man produces both the contrary
effects. For the rational formula is one which applies to both,
though not in the same way, and it is in a soul which possesses an
originative source of movement; so that the soul will start both
processes from the same originative source, having linked them up
with the same thing. And so the things whose potency is according
to a rational formula act contrariwise to the things whose potency
is non-rational; for the products of the former are included under
one originative source, the rational formula.
    It is obvious also that the potency of merely doing a thing or
having it done to one is implied in that of doing it or having it
done well, but the latter is not always implied in the former: for
he who does a thing well must also do it, but he who does it merely
need not also do it well.
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3
    There are some who say, as the Megaric school does, that a thing
‘can’ act only when it is acting, and when it is not acting it
‘cannot’ act, e.g. that he who is not building cannot build, but
only he who is building, when he is building; and so in all other
cases. It is not hard to see the absurdities that attend this
view.
    For it is clear that on this view a man will not be a builder
unless he is building (for to be a builder is to be able to build),
and