 Central to the early modern discussion of self-consciousness are Descartes’ assertions, in the second of his Meditations, that “I am, I exist, is necessarily true whenever it is put forward by me or conceived in my mind” (Descartes 1641: 80), and, in both his Discourse and Principles, that “I think, therefore I am”, or “cogito ergo sum” (Descartes 1637: 36, and Descartes 1644: 162; see the discussion of Reflection in the entry on  seventeenth century theories of consciousness).  The cogito, which was anticipated by Augustine (On the Trinity 10.10; Pasnau 2002: ch. 11), embodies two elements of self-awareness—awareness that one is thinking and awareness that one exists—that play a foundational role in Descartes’ epistemological project. As such, it is crucial for Descartes that the cogito is something of which we can be absolutely certain. But whilst most commentators are happy to agree that both “I am thinking” and “I exist” are indubitable, there is a great deal of debate over the grounds for such certainty and over the form of the cogito itself (Hintikka 1962; Wilson 1978: ch. 2, §2; B. Williams 1978: ch. 3; Markie 1992). Of particular concern is the question whether these two propositions are known by inference or non-inferentially, e.g., by intuition, an issue that echoes the medieval debates concerning whether one can be said to perceive oneself.