方法序説
Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences () is a philosophical and autobiographical treatise published by René Descartes in 1637. It is best known as the source of the famous quotation "Je pense, donc je suis" ("I think, therefore I am", or "I am thinking, therefore I exist"), which occurs in Part IV of the work. A similar argument, without this precise wording, is found in "Meditations on First Philosophy" (1641), and a Latin version of the same statement "Cogito, ergo sum" is found in "Principles of Philosophy" (1644).