colours perceived. 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.

PHIL. Howl is light then a substance?

HYL. . 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.

PHIL. It seems then the light doth no more than shake the optic nerves.

HYL. Nothing else.

PHIL. And consequent to each particular motion of the nerves, the mind
is affected with a sensation, which is some particular colour.

HYL. Right.

PHIL. And these sensations have no existence without the mind.