Plato at least cannot even explain what it is that he sometimes thinks
to be the source of motion, i.e., that which moves itself; for
according to him the soul is posterior to motion and coeval with the
Now to suppose that
potentiality is prior to actuality is in one sense right and in
another wrong; we have explained2
actuality is prior is testified by Anaxagoras (since mind is
actuality), and by Empedocles with his theory of Love and Strife, and
by those who hold that motion is eternal, e.g. Leucippus.
Therefore Chaos or Night did not endure for
an unlimited time, but the same things have always existed, either
passing through a cycle or in accordance with some other
principle—that is, if actuality is prior to
there is a regular cycle, there must be something3
which remains always active in the
same way; but if there is to be generation and destruction, there must
be something else4
which is always active in two
different ways. Therefore this must be active in one way
independently, and in the other in virtue of something else, i.e.
either of some third active principle or of the first.It must, then, be in virtue of
the first; for this is in turn the cause both of the third and of the
second. Therefore the first is preferable, since it was the cause of
perpetual regular motion, and something else was the cause of variety;
and obviously both together make up the cause of perpetual variety.
Now this is just what actually characterizes motions; therefore why
need we seek any further principles?
Since (a) this is
a possible explanation, and (b) if it is not true, we shall have to
regard everything as coming from "Night"5
and "all things together" and "not-being,"6
these difficulties may be considered to be solved.
There is something which is eternally moved with an unceasing motion,
and that circular motion. This is evident not merely in theory, but in
fact. Therefore the "ultimate heaven" must be eternal. Then there is
also something which moves it.And since that which is moved while it moves
is intermediate, there is something which moves without being moved;
something eternal which is both substance and actuality.
Now it moves in the following manner. The
object of desire and the object of thought move without being moved.
The primary objects of desire and thought are the same. For it is the
apparent good that is the object of appetite, and the real good that
is the object of the rational will.7
Desire is the result of opinion
rather than opinion that of desire; it is the act of thinking that is
thought is moved by the intelligible, and one of the series of
intelligible. In this series substance stands first, and of substance
that which is simple and exists actually. (The one and the simple are
not the same; for one signifies a measure,9
whereas "simple" means that the subject itself
is in a certain state.)But
the Good, and that which is in itself desirable, are also in the same