In this book, the author defends a unified externalists account of propositional attitudes and reference, and formalizes this view within possible world semantics. He establishes a link between philosophical analyses of intentionality and reference and formal semantic theories of discourse representation and context change. Stalnakerian diagonalization plays an important role here. Anaphora are treated as referential expressions, while presupposition is seen as a propositional attitude. The relation between belief change and the semantic analyses of conditional sentences and evidential (knowledge) and buletic (desire) propositional attitudes is discussed extensively.