On The Soul
We have already pointed out that there is no necessity that what
originates movement should itself be moved. There are two senses in
which anything may be moved-either (a) indirectly, owing to
something other than itself, or (b) directly, owing to itself.
Things are 'indirectly moved' which are moved as being contained in
something which is moved, e.g. sailors in a ship, for they are moved
in a different sense from that in which the ship is moved; the ship is
'directly moved', they are 'indirectly moved', because they are in a
moving vessel. This is clear if we consider their limbs; the
movement proper to the legs (and so to man) is walking, and in this
case the sailors tare not walking. Recognizing the double sense of
'being moved', what we have to consider now is whether the soul is
'directly moved' and participates in such direct movement.
There are four species of movement-locomotion, alteration,
diminution, growth; consequently if the soul is moved, it must be
moved with one or several or all of these species of movement. Now
if its movement is not incidental, there must be a movement natural to
it, and, if so, as all the species enumerated involve place, place
must be natural to it. But if the essence of soul be to move itself,
its being moved cannot be incidental to-as it is to what is white or
three cubits long; they too can be moved, but only incidentally-what
is moved is that of which 'white' and 'three cubits long' are the
attributes, the body in which they inhere; hence they have no place:
but if the soul naturally partakes in movement, it follows that it
must have a place.
Further, if there be a movement natural to the soul, there must be a
counter-movement unnatural to it, and conversely. The same applies
to rest as well as to movement; for the terminus ad quem of a
thing's natural movement is the place of its natural rest, and
similarly the terminus ad quem of its enforced movement is the place
of its enforced rest. But what meaning can be attached to enforced
movements or rests of the soul, it is difficult even to imagine.