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be eternal movement, since that which is potentially may possibly
not be. There must, then, be such a principle, whose very essence is
actuality. Further, then, these substances must be without matter; for
they must be eternal, if anything is eternal. Therefore they must be
Yet there is a difficulty; for it is thought that everything
that acts is able to act, but that not everything that is able to
act acts, so that the potency is prior. But if this is so, nothing
that is need be; for it is possible for all things to be capable of
existing but not yet to exist.
Yet if we follow the theologians who generate the world from
night, or the natural philosophers who say that 'all things were
together', the same impossible result ensues. For how will there be
movement, if there is no actually existing cause? Wood will surely not
move itself-the carpenter's art must act on it; nor will the menstrual
blood nor the earth set themselves in motion, but the seeds must act
on the earth and the semen on the menstrual blood.
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?

Since (1) this is a possible account of the matter, and (2) if
it were not true, the world would have proceeded out of night and 'all
things together' and out of non-being, these difficulties may be taken
as solved. There is, then, something which is always moved with an
unceasing motion, which is motion in a circle; and this is plain not
in theory only but in fact. Therefore the first heaven must be
eternal. There is therefore also something which moves it. And since
that which moves and is moved is intermediate, there is something
which moves without being moved, being eternal, substance, and
actuality. And the object of desire and the object of thought move
in this way; they move without being moved. The primary objects of
desire and of thought are the same. For the apparent good is the
object of appetite, and the real good is the primary object of

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