This work presents a radical new view of the function of the brain and nervous system. It suggests that the nervous system in each individual operates as a selective system resembling natural selection in evolution but operating different mechanisms. By providing a fundamental neural basis for categorization of the things of the world it unifies perception, action and learning. This theory revises our view of memory, considering it as a dynamic process of recategorization which has implications for the various psychological states from attention to dreaming. It will stimulate discussion about the mind-body problem, the origins of knowledge and the perceptual bases of language. The author won the 1972 Nobel Prize for Physiology of Medicine.