Descartes' works include "Musicae Compendium," "The World," "Man," "Dioptrics," "The Meteors," "Geometry," "Discourse on the Method," "The Meditations," "Principles of Philosophy," "The Search for Truth," "The Description of the Human Body" and "Passions of the Soul." Some were published posthumously as were some essays and letters that were lesser known.
Descartes was a French scholar, mathematician and philosopher who lived from 1596 to 1650. He is credited as the father of modern Western philosophy, analytic geometry and the Cartesian coordinate system (named for the Latin translation of Descartes' name). He also played a key role in the scientific revolution of Europe that subsequently lead to the social movement of the Enlightenment era.
The quote "I think therefore I am," remains the hallmark of Descartes' philosophy. It appeared for the first time in "Discourse on the Method," but was also referenced in later works. Descartes posited that because man can err in reasoning or be mislead by his senses, he must strip away all he believed was real and view it as merely an illusion. Inferring, however, that there is a being, a man who is capable of perceiving or pondering himself and his environment at all was undeniable proof that he, at the very least, existed. This proof became a pillar for many of Descartes' later arguments.