Sources of knowledge:

1
Epistemology the study of the nature, source,
limits, and justification of knowledge

• 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

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., 235) 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 distinctlythat 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 ones ability to
  reason is to assume exactly what we are trying to
  prove (the Cartesian Circle)