The Complete Aristotle (eng.)

matter as well as one of actuality;
e.g. the circle is ‘a plane figure’. But of the things which have
no matter, either intelligible or perceptible, each is by its
nature essentially a kind of unity, as it is essentially a kind of
being-individual substance, quality, or quantity (and so neither
‘existent’ nor ‘one’ is present in their definitions), and the
essence of each of them is by its very nature a kind of unity as it
is a kind of being-and so none of these has any reason outside
itself, for being one, nor for being a kind of being; for each is
by its nature a kind of being and a kind of unity, not as being in
the genus ‘being’ or ‘one’ nor in the sense that being and unity
can exist apart from particulars.
    Owing to the difficulty about unity some speak of
‘participation’, and raise the question, what is the cause of
participation and what is it to participate; and others speak of
‘communion’, as Lycophron says knowledge is a communion of knowing
with the soul; and others say life is a ‘composition’ or
‘connexion’ of soul with body. Yet the same account applies to all
cases; for being healthy, too, will on this showing be either a
‘communion’ or a ‘connexion’ or a ‘composition’ of soul and health,
and the fact that the bronze is a triangle will be a ‘composition’
of bronze and triangle, and the fact that a thing is white will be
a ‘composition’ of surface and whiteness. The reason is that people
look for a unifying formula, and a difference, between potency and
complete reality. But, as has been said, the proximate matter and
the form are one and the same thing, the one potentially, and the
other actually. Therefore it is like asking 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
    Translated by W. D. Ross
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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