One of the foundation-stones of modern philosophy. Descartes was prepared to go to any lengths in his search for certainty - even to deny those things that seemed most self-evident. He set out to dismantle and then reconstruct the idea of the individual self and its existence. In doing so, Descartes developed a language of subjectivity that has lasted to this day, and he also took his first steps towards the view that would eventually be expressed in the epigram Cogito, ergo sum ('I think, therefore I am'), one of modern philosophy's most famous - and most fiercely contested - claims.