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 perishing unnatural (for growing old is natural), nor do
we observe one becoming to be natural and another unnatural. We answer
that if what happens under violence is unnatural, then violent
perishing is unnatural and as such contrary to natural perishing.
Are there then also some becomings that are violent and not the result
of natural necessity, and are therefore contrary to natural becomings,
and violent increases and decreases, e.g. the rapid growth to maturity
of profligates and the rapid ripening of seeds even when not packed
close in the earth? And how is it with alterations? Surely just the
same: we may say that some alterations are violent while others are
natural, e.g. patients alter naturally or unnaturally according as
they throw off fevers on the critical days or not. But, it may be
objected, then we shall have perishings contrary to one another, not
to becoming. Certainly: and why should not this in a sense be so? Thus
it is so if one perishing is pleasant and another painful: and so
one perishing will be contrary to another not in an unqualified sense,
but in so far as one has this quality and the other that.

    Now motions and states of rest universally exhibit contrariety in
the manner described above, e.g. upward motion and rest above are
respectively contrary to downward motion and rest below, these being
instances of local contrariety; and upward locomotion belongs
naturally to fire and downward to earth, i.e. the locomotions of the
two are contrary to each other. And again, fire moves up naturally and
down unnaturally: and its natural motion is certainly contrary to
its unnatural motion. Similarly with remaining: remaining above is
contrary to motion from above downwards, and to earth this remaining
comes unnaturally, this motion naturally. So the unnatural remaining
of a thing is contrary to its natural motion, just as we find a
similar contrariety in the motion of the same thing: one of its
motions, the upward or the downward, will be natural, the other
unnatural.

    Here, however, the question arises, has every state of rest that
is not permanent a becoming, and is this becoming a coming to a
standstill? If so, there must be a becoming of that which is at rest
unnaturally, e.g. of earth at rest above: and therefore this earth
during the time that it was being carried violently upward was
coming to a standstill. But whereas the velocity of that which comes
to a standstill seems always to increase, the velocity of that which
is carried violently seems always to decrease: so it will he in a
state of rest without having become so. Moreover "coming to a
standstill" is generally recognized to be identical or at least
concomitant with the locomotion of a thing to its proper place.

    There is also another difficulty involved in the view that remaining
in a particular place is contrary to motion from that place. For
when a thing is moving from or discarding something, it still
appears to have that which is being discarded, so that if a state of
rest is itself contrary to the motion from the state of rest to its
contrary, the contraries rest and motion will be simultaneously
predicable of the same thing. May we not say, however, that in so
far as the thing is still stationary it is in a state of rest in a
qualified sense? For, in fact, whenever a thing is in motion, part
of it is at the starting-point while part is at the goal to which it
is changing: and consequently a motion finds its true contrary
rather in another motion than in a state of rest.

    With regard to motion and rest, then, we have now explained in
what sense each of them is one and under what conditions they
exhibit contrariety.

    [With regard to coming to a standstill the question may be raised
whether there is an opposite state of rest to unnatural as well as
to natural motions. It would be absurd if this were not the case:
for a thing may remain still merely under violence: thus we shall have
a thing being in a non-permanent state of rest without having become
so. But it is clear that it must be the case: for just as there is
unnatural motion, so, too, a thing may be in an unnatural state of
rest. Further, some things have a natural and an unnatural motion,
e.g. fire has a natural upward motion and an unnatural downward
motion: is it, then, this unnatural downward motion or is it the
natural downward motion of earth that is contrary to the natural
upward motion? Surely it is clear that both are contrary to it
though not in the same sense: the natural motion of earth is
contrary inasmuch as the motion of fire is also natural, whereas the
upward motion of fire as being natural is contrary to the downward
motion of fire as being unnatural. The same is true of the
corresponding cases of remaining. But there would seem to be a sense
in which a state of rest and a motion are opposites.]