In the third volume in the Rutgers Lectures in Philosophy series, distinguished philosopher Robert Stalnaker here offers a defense of an ontology of propositions, and of some logical resources for representing them. He offers an austere formulation of a theory of propositions in a first-order extensional logic, but then uses the commitments of this theory to justify an enrichment to modal logic as an appropriate framework for regimented languages that are constructed to represent any of our scientific and philosophical commitments. His book adopts a self-consciously neo-Quinean methodology, and argues that the theory that is developed helps to motivate and clarify Quine's naturalistic metaphysical picture.