The Complete Aristotle (eng.)

Further even if it
acts, this will not be enough, if its essence is potency; for there
will not 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 actuality.
    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?
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7
    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 rational wish. But desire is consequent on
opinion rather than opinion on desire; for the thinking is the
starting-point. And thought is moved by the object of thought, and
one of the two columns of opposites is in itself the object of
thought; and in this, substance is first, and in substance, that
which is