[
1071b]
[1]
Thus
we have stated what the principles of sensible things are, and how
many they are, and in what sense they are the same and in what sense
different.
Since we have seen
1 that there are three kinds of
substance, two of which are natural and one immutable, we must now
discuss the last named and show that there must be some substance
which is eternal and immutable. Substances are the primary reality,
and if they are all perishable, everything is perishable. But motion
cannot be either generated or destroyed, for it always existed
2; nor can time, because there
can be no priority or posteriority if there is no time.
3 Hence as time is continuous,
so too is motion; for time is either identical with motion or an
affection of it.
4 But there is no
continuous motion except that which is spatial, of spatial motion only
that which is circular.
5But even if we
are to suppose that there is something which is kinetic and productive
although it does not actually move or produce, there will not
necessarily be motion; for that which has a potentiality may not
actualize it.Thus it will
not help matters if we posit eternal substances, as do the exponents
of the Forms, unless there is in them some principle which can cause
change.
6 And even this is not enough, nor is it
enough if there is another substance besides the Forms; for unless it
actually functions there will not be motion.And it will still not be enough even if it
does function, if its essence is potentiality; for there will not be
eternal motion, since that which exists potentially may not
exist.
[20]
Therefore there
must be a principle of this kind whose essence is actuality.
Furthermore these substances
7 must be
immaterial; for they must be eternal if anything is. Therefore they
are actuality.
There is a difficulty, however; for it seems
that everything which actually functions has a potentiality, whereas
not everything which has a potentiality actually functions; so that
potentiality is prior. But if this is so, there need be no reality;
for everything may be capable of existing, but not yet
existent.Yet if we
accept the statements of the cosmologists who generate everything from
Night,
8 or the doctrine of the physicists that "all
things were together,"
9 we have the same
impossibility; for how can there be motion if there is no actual
cause? Wood will not move itself—carpentry must act upon it;
nor will the menses or the earth move themselves—the seeds
must act upon the earth, and the semen on the menses.Hence some, e.g. Leucippus
10
and Plato,
11 posit an
eternal actuality, for they say that there is always motion; but why
there is, and what it is, they do not say; nor, if it moves in this or
that particular way, what the cause is. For nothing is moved at
haphazard, but in every case there must be some reason present; as in
point of fact things are moved in one way by nature, and in another by
force or mind or some other agent. And further, what kind of motion is
primary? For this is an extremely important point.