The Identity Thesis, proposed by Reid for the case of sensations, and extended by Brentano to conscious states generally, says that a state is conscious iff it is identical with introspective knowledge of its own instantiation. The Thesis offers simple explanations of a number of puzzling features of introspective self-knowledge, and unites the problems of introspection, consciousness and knowledge in the single problem of the metaphysical nature of conscious states. It does nothing to solve the latter problem, but it does entail that a functionalist account of consciousness is incomplete without an exact functionalist definition of knowledge itself.