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...