The Complete Aristotle (eng.)

motion. And in
all cases of a thing that has no contrary we have as contraries
change from and change to the same thing. Thus coming to be is
contrary to ceasing to be, and losing to gaining. But these are
changes and not motions. And wherever a pair of contraries admit of
an intermediate, motions to that intermediate must be held to be in
a sense motions to one or other of the contraries: for the
intermediate serves as a contrary for the purposes of the motion,
in whichever direction the change may be, e.g. grey in a motion
from grey to white takes the place of black as starting-point, in a
motion from white to grey it takes the place of black as goal, and
in a motion from black to grey it takes the place of white as goal:
for the middle is opposed in a sense to either of the extremes, as
has been said above. Thus we see that two motions are contrary to
each other only when one is a motion from a contrary to the
opposite contrary and the other is a motion from the latter to the
former.
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6
    But since a motion appears to have contrary to it not only
another motion but also a state of rest, we must determine how this
is so. A motion has for its contrary in the strict sense of the
term another motion, but it also has for an opposite a state of
rest (for rest is the privation of motion and the privation of
anything may be called its contrary), and motion of one kind has
for its opposite rest of that kind, e.g. local motion has local
rest. This statement, however, needs further qualification: there
remains the question, is the opposite of remaining at a particular
place motion from or motion to that place? It is surely clear that
since there are two subjects between which motion takes place,
motion from one of these (A) to its contrary (B) has for its
opposite remaining in A while the reverse motion has for its
opposite remaining in B. At the same time these two are also
contrary to each other: for it would be absurd to suppose that
there are contrary motions and not opposite states of rest. States
of rest in contraries are opposed. To take an example, a state of
rest in health is (1) contrary to a state of rest in disease, and
(2) the motion to which it is contrary is that from health to
disease. For (2) it would be absurd that its contrary motion should
be that from disease to health, since motion to that in which a
thing is at rest is rather a coming to rest, the coming to rest
being found to come into being simultaneously with the motion; and
one of these two motions it must be. And (1) rest in whiteness is
of course not contrary to rest in health.
    Of all things that have no contraries there are opposite changes
(viz. change from the thing and change to the thing, e.g. change
from being and change to being), but no motion. So, too, of such
things there is no remaining though there is absence of change.
Should there be a particular subject, absence of change in its
being will be contrary to absence of change in its not-being. And
here a difficulty may be raised: if not-being is not a particular
something, what is it, it may be asked, that is contrary to absence
of change in a thing’s being? and is this absence of change a state
of rest? If it is, then either it is not true that every state of
rest is contrary to a motion or else coming to be and ceasing to be
are motion. It is clear then that, since we exclude these from
among motions, we must not say that this absence of change is a
state of rest: we must say that it is similar to a state of rest
and call it absence of change. And it will have for its contrary
either nothing or absence of change in the thing’s not-being, or
the ceasing to be of the thing: for such ceasing to be is change
from it and the thing’s coming to be is change to it.
    Again, a further difficulty may be raised. How is it, it may be
asked, that whereas in local change both remaining and moving may
be natural or unnatural, in the other changes this is not so? e.g.
alteration is not now natural and now unnatural, for convalescence
is no more natural or unnatural than falling ill, whitening no more
natural or unnatural than blackening; so, too, with increase and
decrease: these are not contrary to each other in the sense that
either of them is natural while the other is unnatural, nor is one
increase contrary to another in this sense; and the same account
may be given of becoming and perishing: it is not true that
becoming is natural and