When William of Ockham lectured on Lombard’s Sentences in 1317-1319, he articulated a new theory of knowledge. Its reception by fourteenth-century scholars was, however, largely negative, for it conflicted with technical accounts of vision and with their interprations of Duns Scotus. This study begins with Roger Bacon, a major source for later scholastics’ efforts to tie a complex of semantic and optical explanations together into an account of concept formation, truth and the acquisition of certitude. After considering the challenges of Peter Olivi and Henry of Ghent, Part I concludes with a discussion of Scotus’s epistemology. Part II explores the alternative theories of Peter Aureol and William of Ockham. Part III traces the impact of Scotus, and then of Aureol, on Oxford thought in the years of Ockham’s early audience, culminating with the views of Adam Wodeham. Part IV concerns Aureol’s intellectual legacy at Paris, the introduction of Wodeham’s thought there, and Autrecourt’s controversies.