“Hylas. 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.

Philonous. It seems then, the light does no more than shake the optic nerves.

Hylas. Nothing else.

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

Hylas. Right.

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

Hylas. They have not.”

—George Berkeley, from Three Dialogues between Hylas and Philonous, 1713.