Besides, allowing there are colours on external objects, yet, how
is it possible for us to perceive them? For no external body
affects the mind, unless it acts first on our organs of sense.
But the only action of bodies is motion; and motion cannot be
communicated otherwise than by impulse. A distant object
therefore cannot act on the eye; nor consequently make itself or
its properties perceivable to the soul. Whence it plainly follows
that it is immediately some contiguous substance, which,
operating on the eye, occasions a perception of colours: and such
is light.
. Howl is light then a substance?
.. I tell you, Philonous, external light is nothing but
a thin fluid substance, whose minute particles being agitated
with a brisk motion, and in various manners reflected from the
different surfaces of outward objects to the eyes, communicate
different motions to the optic nerves; which, being propagated to
the brain, cause therein various impressions; and these are
attended with the sensations of red, blue, yellow, &c.
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