This dissertation defends the claim that Walter Burley (died c. 1345) develops a sophisticated account of the proposition, where a proposition is an entity fulfilling various semantic and epistemic roles - first and foremost, to be the primary bearer of truth value. In doing so, it attempts to correct longstanding misconceptions about a key aspect of Burley's philosophy. In contrast to most approaches in the literature, which claim that Burley is principally concerned with the nature of states of affairs, this dissertation argues that he is concerned rather with the metaphysics of the proposition. Motivated by two deeper semantic commitments, Burley argues that a proposition is a complex entity composed of "things outside the soul" (res extra animam). The proposition that Barack Obama is a human, for example, is composed of Barack Obama and the property of humanity. Moreover, encouraged by the view that truth is fundamentally a matter of the mind's correspondence to the world, Burley argues that cognitive agents have the ability to arrange things together into propositions, where that arrangement is the exercise of a capacity to represent the world's being some way or other - for example, to represent that Barack Obama is a human. Consequently, on Burley's view, propositions are structured; they are complex, truth-conditional entities composed of things by mental acts. This dissertation situates that account of the proposition within Burley's wider account of thinking and demonstrative science. To that end, it argues that Burley develops a theory of mental language, according to which thinking is fundamentally a matter of mind's use of sentences composed of concepts, that is, of mental representations. Burley's commitment to mental language accomplishes a number of goals. Most significantly, it helps him integrate his account of the proposition into his account of demonstrative science, by resolving tensions brought on by the opacity of belief. Finally, this dissertation examines the evolution of Burley's account of the truth conditions of the proposition, arguing that the final account of those conditions involves the radical claim that things - the referents of categorematic expressions in natural and mental language - must have certain intrinsic semantic properties.