Sir Isaac Newton, a contemporary of Locke’s, knew that the rich perceptual experiences we have are created in our brains, not out there in the world. He wrote that the light waves illuminating a blue sky are not themselves blue—they only appear blue because our retina and cortex interpret light of a particular frequency, 650 terahertz, as blue. Blueness is an interpretation that we place on the world, not something that is objectively there.
