1. Sources of knowledge: –Sense experience (empiricism) –Reasoning alone (rationalism) We truly know only that of which we are certain (a priori). Since sense experience (a posteriori knowledge) cannot guarantee certainty, reason alone must be the means for getting knowledge Epistemology: the study of the nature, source, limits, and justification of knowledge

2. Cartesian Doubt Sense experiences are often wrong I might be wrong about whether I have a body or if there is a world apart from my imagination (it may be a dream) I might be wrong even about whether my reasoning abilities (e.g., 2+3=5) can be trusted (evil genius); so I should suspend judgment René Descartes (1596-1650)

3. Descartes’ Escape from Doubt I cannot doubt that I am doubting (thinking). All other knowledge is based on the indubitable foundation that I exist: I think, therefore I exist (cogito ergo sum) I know I exist imperfectly only by first knowing perfect existence (God); and as a perfect being, God would not deceive me or allow me to be deceived when I know something (e.g., wax) clearly and distinctly—that is, as an intelligible (e.g., mathematical) object

4. Objections to Descartes’ Method of Doubt To doubt everything, we must doubt whether we are truly doubting, and that requires us to assume a public world of language users Limiting knowledge only to what we know with certainty is too restrictive: we often know things not based on indubitable foundations To reason that God guarantees one’s ability to reason is to assume exactly what we are trying to prove (the Cartesian Circle)