The very important difference between what is physically present in the world and what is expe- rienced psychologically had been recognized and agonized over for centuries. This was the distinction that had caused Galileo to conclude that a science of psychology was impossible and Hume to conclude that we could know nothing about the physical world with certainty. Kant amplified this distinction when he claimed that the mind embellished sensory experience, and Helmholtz reached the same con- clusion with his concept of unconscious inference.
With advances in science, much had been learned about the physical world—including about the physical stimulation of the sense receptors, which convert that stimulation into nerve impulses, and about the brain structures where those impulses terminate. There was never much doubt about the existence of consciousness; the problem was in determining what we were conscious of and what caused that consciousness. By now it was widely believed that conscious sensations were triggered by brain processes, which themselves were initiated by sense reception. But the question remained: How are the two domains (conscious mental events and the physiological processes of our sensory system) related?