g.
those capable of forward movement must have them; for, if they are
to survive, they must perceive not only by immediate contact but
also at a distance from the object. This will be possible if they
can perceive through a medium, the medium being affected and moved
by the perceptible object, and the animal by the medium. just as
that which produces local movement causes a change extending to a
certain point, and that which gave an impulse causes another to
produce a new impulse so that the movement traverses a medium the
first mover impelling without being impelled, the last moved being
impelled without impelling, while the medium (or media, for there
are many) is both-so is it also in the case of alteration, except that
the agent produces produces it without the patient's changing its
place. Thus if an object is dipped into wax, the movement goes on
until submersion has taken place, and in stone it goes no distance
at all, while in water the disturbance goes far beyond the object
dipped: in air the disturbance is propagated farthest of all, the
air acting and being acted upon, so long as it maintains an unbroken
unity. That is why in the case of reflection it is better, instead
of saying that the sight issues from the eye and is reflected, to
say that the air, so long as it remains one, is affected by the
shape and colour. On a smooth surface the air possesses unity; hence
it is that it in turn sets the sight in motion, just as if the
impression on the wax were transmitted as far as the wax extends.
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