This dissertation argues that fourteenth-century debates about the metaphysics of cognitio were debates about the metaphysics of conscious experience. These medieval debates about conscious experience have been overlooked in the rich secondary literature on medieval philosophy of mind. I call the shift by which conscious experience came to be at the heart of medieval debates about the metaphysics of cognition the "Subjective Turn." The dissertation traces the Subjective Turn by examining the thought of several influential scholastics within their contemporary context, focusing especially on Peter Auriol, Adam Wodeham, William Crathorn, Nicholas of Autrecourt, and John of Ripa. I argue that two deep problems about the nature of conscious experience were central to the Subjective Turn. One is a problem about intentionality: How does the intentionality of experience, of the way things appear to a subject, relate to the intentionality of what medieval philosophers called similitudines, mental likenesses that represent objects in the world? The other is a problem about consciousness: How does the phenomenon of objects appearing to subjects relate to the phenomenon of subjects being aware of or experiencing their own experiences? The dissertation examines the responses to these problems that emerged during the Subjective Turn.