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 for thought". In other words, "the only claim that is indubitable here is the agent-independent claim that there is cognitive activity present". The objection, as presented by Georg Lichtenberg, is that rather than supposing an entity that is thinking, Descartes should have said: "thinking is occurring." That is, whatever the force of the "cogito", Descartes draws too much from it; the existence of a thinking thing, the reference of the "I," is more than the "cogito" can justify. Friedrich Nietzsche criticized the phrase in that it presupposes that there is an "I", that there is such an activity as "thinking",