Discourse on the Method of Rightly Conducting the Reason, and Seeking Truth in the Sciences. By Rene Descartes.
This work is one of the most influential in history. The famous phrase, "COGITO ERGO SUM" (I think, therefore I am) is a central theme. Descartes' beliefs on the dual nature of mind and body, and his emphasis on the role of doubt in all inquiry, formed the basis for centuries of science and social thought.