tentiality and
actuality. Potentiality is the same as matter, actuality as form. For
matter is potentially everything. It may become everything. It is not
actually anything. It is a mere potentiality, or capacity of becoming
something. But whatever gives it definiteness as a this or that,
whatever makes it an actual thing, is its form. Thus the actuality of
a thing is simply its form.

Aristotle claims, by means of the antithesis of potentiality and
actuality, to have solved the ancient problem of becoming, a riddle,
propounded by the Eleatics, which had never ceased to trouble Greek
thinkers. How is becoming possible? For being to pass into being is
not becoming, for it involves no change, and for not-being to pass
into being is impossible, since something cannot come out of nothing.
For Aristotle, the sharp line drawn between not-being and being does
not exist. For these absolute terms he substitutes the relative terms
potentiality and actuality, which shade off into each other.
Potentiality in his philosophy takes the place of not-being in
previous systems. It solves the riddle because it is not an absolute
not-being. It is {280} not-being inasmuch as it is actually nothing,
but it is being because it is potential being. Becoming, therefore,
does not involve the impossible leap from nothing to something. It
involves the transition from potential to actual being. All change,
all motion, is thus the passage of potentiality into actuality, of
matter into form.

Since matter is in itself nothing, a bare unrealised capacity, while
form is actuality, the completed and perfected being, it follows that
form is something higher than matter. But matter is what becomes form.
In order of time, therefore, matter is earlier, form later. But in
order of thought, and in reality, it is otherwise. For when we say
that matter is the potentiality of what it is to become, this implies
that what it is to become is already present in it ideally and
potentially, though not actually. The end, therefore, is already
present in the beginning. The oak is in the acorn, ideally, otherwise
the oak could never come out of it. And since all becoming is towards
the end, and would not take place but for the end, the end is the
operative principle and true cause of becoming. Motion is produced not
by a mechanical propulsive force, pushing from behind, so to speak,
but by an ideal attractive force, drawing the thing towards its end,
as a piece of iron