23. For sense-objects, which receive their impulse from without,
movement is a stimulus which agitates them, excites them, presses them,
prevents them from slumbering in inertia, from remaining the same, and
makes them present an image of life by their agitation and continual
mutations. Besides, one must not confuse the things that move with
movement; walking is not the feet, but an actualization of the power
connected with the feet. Now as this power is invisible, we perceive
only the agitation of the feet; we see that their present state is
quite different from that in which they would have been, had they
remained in place, and that they have some addition, which however, is
invisible. Thus, being united to objects other than itself, the power
is perceived only accidentally, because one notices that the feet
change place, and do not rest. Likewise, alteration in the altered
object, is recognized only by failure to discover in it the same
quality as before.