"Cogito Ergo Sum". ("I think, therefore I am.")

René Descartes was born on March 31st, 1596 in the town of La Haye in the south of France, the son of  Joachim Descartes, a Councilor in Parliament and and intellectual who made certain to provide a good learning environment for his son.  In 1606, at the age of 8, René attended the Jesuitcollege of Henri IV in La Flèche, where he studied literature, grammar, science, and mathematics.  In 1614, he left La Flèche to study civil and cannon Law at Poitiers.  In 1616, he received his baccalaureate and licentiate degrees in Law.  Aside from his Law degrees, Descartes also spent time studying philosophy, theology, and medicine.
After a short stay in the military, Descartes went on to lead a quiet life, continuing his intellectual pursuits, writing philosophical essays, and exploring the world of science and mathematics.  In 1637, he published "geometry", in which his combination of algebra and geometry gave birth to analytical geometry, better known as Cartesian geometry.
But the most important contribution Descartes made were his philosophical writings; Descartes, who was convinced that science and mathematics could be used to explain everything in nature, was the first to describe the physical universe in terms of matter and motion, seeing the universe a as giant mathematically designed engine.  Descartes wrote three important texts:   Discourse on the Method of Rightly Conducting the Reason and Seeking Truth in the Sciences, Meditations on First Philosophy, and Principles of Philosophy.
René Descartes had always been a frail individual, and he would usually spend most of his mornings in bed, where he did most of his thinking, fresh from dreams in which he often had his revelations.  In his latter years, Descartes had to relocate toSweden to tutor Queen Christina in philosophy.  Unfortunately, the Queen was an early riser who wanted her lessons at 5:00 o'clock in the morning.  This new schedule did not help Descartes fragile health, and he contracted pneumonia, from which he died on February 11, 1650 at the age of 54.

Taken from:

Also see:
http://www.maths.tcd.ie/pub/HistMath/People/Descartes/RouseBall/RB_Descartes.html

http://www.radicalacademy.com/phildescartes1.htm

http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Descartes.html