Foundationalism and distinct. Geometrical truths are also certain and indubitable; Descartes thus attempted to find truths which were clear and distinct because they would be indubitably true and a suitable foundation for knowledge. His method was to question all of his beliefs until he reached something clear and distinct that was indubitably true. The result was his "cogito ergo sum" – 'I think therefore I am', or the belief that he was thinking – as his indubitable belief suitable as a foundation for knowledge. This resolved Descartes' problem of the Evil Demon – the possibility that he was being deceived by
Cogito, ergo sum itself is good and pleasant (...) and if one who sees is conscious that he sees, one who hears that he hears, one who walks that he walks and similarly for all the other human activities there is a faculty that is conscious of their exercise, so that whenever we perceive, we are conscious that we perceive, and whenever we think, we are conscious that we think, and to be conscious that we are perceiving or thinking is to be conscious that we exist... ("Nicomachean Ethics", 1170a25 ff.) Augustine of Hippo in "De Civitate Dei" writes "Si […] fallor, sum"