This book develops in detail the simple idea that assertion is the expression of belief. In it the author puts forward a version of 'probabilistic semantics' which acknowledges that we are not perfectly rational, and which offers a significant advance in generality on theories of meaning couched in terms of truth conditions. It promises to challenge a number of entrenched and widespread views about the relations of language and mind. Part I presents a functionalist account of belief, worked through a modified form of decision theory. In Part II the author generates a theory of meaning in terms of 'assertibility conditions', whereby to know the meaning of an assertion is to know the belief it expresses.