Chomsky's doctrine that ordinary speakers tacitly know the syntactic rules of their language is fundamental for linguistics and psychology. But the claim has been treated with suspicion by many philosophers, some of whom have argued that the whole idea rests on quite grotesque conceptual confusions. This book sets out and defends an account of tacit knowledge of linguistic rules, locating the notion against the background of a content based scientific psychology, and exploring its employment in the theory of language acquisition, the study of acquired disorders of language, and the separation of semantic and pragmatic factors in communication. Tacit knowledge is defined as a kind of causal explanatory structure, and the distinction between tacit knowledge and ordinary propositional attitudes is compared with the psychological distinction between special purpose modules and the central cognitive system. The book will be of interest to psychologists and linguists, as well as philosphers.