Hans Joachim Stoerig: Kleine Weltgeschichte der Philosophie (in German): René Descartes (1596-1649): preference for mathematics combined with scepticism against all other sciences; alternating phases of reclusiveness / concentration and adventurous life style; two basic elements of cartesian thinking: God and Soul, but logically discussed; tried to approach philosophy as a kind of universal mathematics, reducing all philosophical principles by deduction from simple basic elements; first awareness that everything that I know or perceive can be wrong; then out of this doubt follows at least the basic assurance of being a thinking entity; this leads to 'cogito ergo sum' (I think, therfore I am) as a fundamental starting point.

Other Links:
Internet Encyclopedia of Philosophy: René Descartes