Metaphysics
by Aristotle
Written circa 350 B.C.
Translated by W. D. Ross
Book XII.6
This is why some suppose eternal actuality-e.g. Leucippus and
Plato; for they say there is always movement. But why and what this
movement is they do say, nor, if the world moves in this way or
that, do they tell us the cause of its doing so. Now nothing is
moved at random, but there must always be something present to move
it; e.g. as a matter of fact a thing moves in one way by nature, and
in another by force or through the influence of reason or something
else. (Further, what sort of movement is primary? This makes a vast
difference.) But again for Plato, at least, it is not permissible to
name here that which he sometimes supposes to be the source of
movement-that which moves itself; for the soul is later, and coeval
with the heavens, according to his account. To suppose potency prior
to actuality, then, is in a sense right, and in a sense not; and we
have specified these senses. That actuality is prior is testified by
Anaxagoras (for his 'reason' is actuality) and by Empedocles in his
doctrine of love and strife, and by those who say that there is always
movement, e.g. Leucippus. Therefore chaos or night did not exist for
an infinite time, but the same things have always existed (either
passing through a cycle of changes or obeying some other law), since
actuality is prior to potency. If, then, there is a constant cycle,
something must always remain, acting in the same way. And if there
is to be generation and destruction, there must be something else
which is always acting in different ways. This must, then, act in
one way in virtue of itself, and in another in virtue of something
else-either of a third agent, therefore, or of the first. Now it
must be in virtue of the first. For otherwise this again causes the
motion both of the second agent and of the third. Therefore it is
better to say 'the first'. For it was the cause of eternal uniformity;
and something else is the cause of variety, and evidently both
together are the cause of eternal variety. This, accordingly, is the
character which the motions actually exhibit. What need then is
there to seek for other principles?