The Complete Aristotle (eng.)

goal but in the manner of
the motion: for in some cases the motion is differentiated by
quickness and slowness: thus if its velocity is uniform a motion is
regular, if not it is irregular. So quickness and slowness are not
species of motion nor do they constitute specific differences of
motion, because this distinction occurs in connexion with all the
distinct species of motion. The same is true of heaviness and
lightness when they refer to the same thing: e.g. they do not
specifically distinguish earth from itself or fire from itself.
Irregular motion, therefore, while in virtue of being continuous it
is one, is so in a lesser degree, as is the case with locomotion in
a broken line: and a lesser degree of something always means an
admixture of its contrary. And since every motion that is one can
be both regular and irregular, motions that are consecutive but not
specifically the same cannot be one and continuous: for how should
a motion composed of alteration and locomotion be regular? If a
motion is to be regular its parts ought to fit one another.
<
    div id="section45" class="section" title="5">
5
    We have further to determine what motions are contrary to each
other, and to determine similarly how it is with rest. And we have
first to decide whether contrary motions are motions respectively
from and to the same thing, e.g. a motion from health and a motion
to health (where the opposition, it would seem, is of the same kind
as that between coming to be and ceasing to be); or motions
respectively from contraries, e.g. a motion from health and a
motion from disease; or motions respectively to contraries, e.g. a
motion to health and a motion to disease; or motions respectively
from a contrary and to the opposite contrary, e.g. a motion from
health and a motion to disease; or motions respectively from a
contrary to the opposite contrary and from the latter to the
former, e.g. a motion from health to disease and a motion from
disease to health: for motions must be contrary to one another in
one or more of these ways, as there is no other way in which they
can be opposed.
    Now motions respectively from a contrary and to the opposite
contrary, e.g. a motion from health and a motion to disease, are
not contrary motions: for they are one and the same. (Yet their
essence is not the same, just as changing from health is different
from changing to disease.) Nor are motion respectively from a
contrary and from the opposite contrary contrary motions, for a
motion from a contrary is at the same time a motion to a contrary
or to an intermediate (of this, however, we shall speak later), but
changing to a contrary rather than changing from a contrary would
seem to be the cause of the contrariety of motions, the latter
being the loss, the former the gain, of contrariness. Moreover,
each several motion takes its name rather from the goal than from
the starting-point of change, e.g. motion to health we call
convalescence, motion to disease sickening. Thus we are left with
motions respectively to contraries, and motions respectively to
contraries from the opposite contraries. Now it would seem that
motions to contraries are at the same time motions from contraries
(though their essence may not be the same; ‘to health’ is distinct,
I mean, from ‘from disease’, and ‘from health’ from ‘to
disease’).
    Since then change differs from motion (motion being change from
a particular subject to a particular subject), it follows that
contrary motions are motions respectively from a contrary to the
opposite contrary and from the latter to the former, e.g. a motion
from health to disease and a motion from disease to health.
Moreover, the consideration of particular examples will also show
what kinds of processes are generally recognized as contrary: thus
falling ill is regarded as contrary to recovering one’s health,
these processes having contrary goals, and being taught as contrary
to being led into error by another, it being possible to acquire
error, like knowledge, either by one’s own agency or by that of
another. Similarly we have upward locomotion and downward
locomotion, which are contrary lengthwise, locomotion to the right
and locomotion to the left, which are contrary breadthwise, and
forward locomotion and backward locomotion, which too are
contraries. On the other hand, a process simply to a contrary, e.g.
that denoted by the expression ‘becoming white’, where no
starting-point is specified, is a change but not a