There are six sorts of movement: generation, destruction, increase,
diminution, alteration, and change of place.
It is evident in all but one case that all these sorts of movement are
distinct each from each. Generation is distinct from destruction,
increase and change of place from diminution, and so on. But in the
case of alteration it may be argued that the process necessarily
implies one or other of the other five sorts of motion. This is not
true, for we may say that all affections, or nearly all, produce in us
an alteration which is distinct from all other sorts of motion, for
that which is affected need not suffer either increase or diminution or
any of the other sorts of motion. Thus alteration is a distinct sort of
motion; for, if it were not, the thing altered would not only be
altered, but would forthwith necessarily suffer increase or diminution
or some one of the other sorts of motion in addition; which as a matter
of fact is not the case. Similarly that which was undergoing the
process of increase or was subject to some other sort of motion would,
if alteration were not a distinct form of motion, necessarily be
subject to alteration also. But there are some things which undergo
increase but yet not alteration. The square, for instance, if a gnomon
is applied to it, undergoes increase but not alteration, and so it is
with all other figures of this sort. Alteration and increase,
therefore, are distinct.
Speaking generally, rest is the contrary of motion. But the different
forms of motion have their own contraries in other forms; thus
destruction is the contrary of generation, diminution of increase, rest
in a place, of change of place. As for this last, change in the reverse
direction would seem to be most truly its contrary; thus motion upwards
is the contrary of motion downwards and vice versa.
In the case of that sort of motion which yet remains, of those that
have been enumerated, it is not easy to state what is its contrary. It
appears to have no contrary, unless one should define the contrary here
also either as ‘rest in its quality’ or as ‘change in the direction of
the contrary quality’, just as we defined the contrary of change of
place either as rest in a place or as change in the reverse direction.
For a thing is altered when change of quality takes place; therefore
either rest in its quality or change in the direction of the contrary
may be called the contrary of this qualitative form of motion. In this
way becoming white is the contrary of becoming black; there is
alteration in the contrary direction, since a change of a qualitative
nature takes place.