After the death of Thomas Aquinas and well into the fourteenth century, medieval philosophers strenuously debated the nature of human cognition and the means by which it is achieved. Ockham took a radical stand in that debate, rejecting the widely accepted view at the heart of Aquinas's account, namely, that cognition is mediated by intelligible and sensible species. (I will explain this technical use of 'species' in Section I.) In a variety of texts, Ockham went to great lengths to argue against not only what many scholars take to be Aquinas's own account but also a revised version of it adopted by Duns Scotus. Why did Ockham take such a stand? Why did he insist on rejecting species as a means of cognition, and what alternative theory did he adopt?

The full and complete answer to these questions would no doubt require a very lengthy book. I want to give only one part of the answer, focusing on Ockham’s theory of the cognition of an individual extramental material object. Even so, for the sake of brevity, I will leave to one side an array of topics that are to some degree relevant. Thus, for example, I will leave largely unexamined Ockham’s response to the optics of his time, which relied on species, and his reinterpretation of certain texts of Aristotle’s, which seem to give species a prominent place. Instead I will focus just on Ockham’s account of an ordinary and comparatively simple cognitive experience. I lift my eyes, look across the room, and see a coffee cup. What is it about me that explains my cognition of the cup for Ockham? And why does he feel impelled to reject Aquinas’s view of the cognitive processes involved in perception of this sort?