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what in general is the cause of unity and of a thing's being one;
for each thing is a unity, and the potential and the actual are
somehow one. Therefore there is no other cause here unless there is
something which caused the movement from potency into actuality. And
all things which have no matter are without qualification
essentially unities.

Book IX
1

WE have treated of that which is primarily and to which all the
other categories of being are referred-i.e. of substance. For it is in
virtue of the concept of substance that the others also are said to
be-quantity and quality and the like; for all will be found to involve
the concept of substance, as we said in the first part of our work.
And since 'being' is in one way divided into individual thing,
quality, and quantity, and is in another way distinguished in
respect of potency and complete reality, and of function, let us now
add a discussion of potency and complete reality. And first let us
explain potency in the strictest sense, which is, however, not the
most useful for our present purpose. For potency and actuality
extend beyond the cases that involve a reference to motion. But when
we have spoken of this first kind, we shall in our discussions of
actuality' explain the other kinds of potency as well.
We have pointed out elsewhere that 'potency' and the word 'can'
have several senses. Of these we may neglect all the potencies that
are so called by an equivocation. For some are called so by analogy,
as in geometry we say one thing is or is not a 'power' of another by
virtue of the presence or absence of some relation between them. But
all potencies that conform to the same type are originative sources of
some kind, and are called potencies in reference to one primary kind
of potency, which is an originative source of change in another
thing or in the thing itself qua other. For one kind is a potency of
being acted on, i.e. the originative source, in the very thing acted
on, of its being passively changed by another thing or by itself qua
other; and another kind is a state of insusceptibility to change for
the worse and to destruction by another thing or by the thing itself
qua other by virtue of an originative source of change. In all these
definitions is implied the formula if potency in the primary
sense.-And again these so-called potencies are potencies either of
merely acting or being acted on, or of acting or being acted on
well, so that even in the formulae of the latter the formulae of the
prior kinds of potency are somehow implied.
Obviously, then, in a sense the potency of acting and of being
acted on is one (for a thing may be 'capable' either because it 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

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