Descartes' proof that his essence is thinking.--Thoughtless brutes.--Descartes' proof that he is essentially a non-material thing.--Behaviorism as a philsosphy of psychology.--The privacy of experience.--Wittgenstein on the nature of mind.--The myth of cognitive processes and structures.--Moore and Wittgenstein and the sense of "I know."--The groundlessness of belief.