The present dissertation concerns cognitive psychology—theories about the nature and mechanism of perception and thought—during the High Middle Ages (1250–1350). Many of the issues at the heart of philosophy of mind today—intentionality, mental representation, the active/passive nature of perception—were also the subject of intense investigation during this period. I provide an analysis of these debates with a special focus on Durand of St.-Pourçain, a contemporary of John Duns Scotus and William of Ockham. Durand was widely recognized as a leading philosopher (...) until the advent of the early modern period, yet his views have been largely neglected in the last century. The aim of my dissertation, then, is to provide a new understanding of Durand’s cognitive psychology and to establish a better picture of developments in cognitive psychology during the period. -/- Most philosophers in the High Middle Ages held, in one form or another, the thesis that most forms of cognition (thought, perception) involve the reception of the form of the object into the mind. Such forms in the mind explain what a given episode of cognition is about, its content. According to what has been called the conformality theory of content, the content of our mental states is fixed by this form in the mind. Durand rejects this thesis, and one of the primary theses that I pursue is that Durand replaces the conformality theory of content with a causal theory of content, according to which the content of our mental states is fixed by its cause. When I think about Felix and not Graycat, this is to be explained not by the fact that I have in my mind the form of Felix and not Graycat, but rather by the fact that Felix and not Graycat caused my thought. -/- This is both a controversial interpretation and, indeed, a controversial theory. It is a controversial interpretation because Durand seems to reject the thesis that objects are the causes of our mental states. In the first half of the present dissertation, I argue that Durand does not reject this thesis but he rejects another nearby thesis: that objects as causes give to us ‘forms’. On Durand’s view, an object causes a mental state even though it does not give to us a new ‘form’. In the second half of the dissertation I defend Durand’s causal theory of content against salient objections to it. (shrink)