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 and the History of Madness at the end of the renaissance and the beginning of the classical age. He then discusses the treatment of madness by Descartes in the "Meditations on First Philosophy". Foucault argues that Descartes refuses to recognise a commonality between himself and people who are mad and uses this dissociation as an excuse not to take madness seriously as a grounds for doubt. For Foucault this exclusion of madness by Descartes leads to a cogito that is more or less arbitrarily self-assured of its own rationality. Derrida argues that Descartes might appear to dismiss madness at the point of the meditation to