'I concluded that I was a substance whose whole essence or nature resides only in thinking, and which, in order to exist, has no need of place and is not dependent on any material thing.' Descartes's A Discourse on the Method of Correctly Conducting One's Reason and Seeking Truth in the Sciences marks a watershed in European thought; in it, the author provides an informal intellectual autobiography in the vernacular for a non-specialist readership, sweeps away all previous philosophical traditions, and sets out in brief his radical new philosophy, which begins with a proof of the existence of the self (the famous 'cogito ergo sum'), next deduces from it the existence and nature of God, and ends by offering a radical new account of the physical world and of human and animal nature. This new translation is accompanied by a substantial introductory essay which draws on Descartes's correspondence to examine his motivation and the impact of his great work on his contemporaries. Detailed notes explain his philosophical terminology and ideas.