While serving in the Bavarian army in 1619, he conceived it to be his task to establish a basis for human knowledge secure from scepticism. He formulated a rational scheme of knowledge in his work 'Meditations on First Philosophy'.

His enquiry begins by claiming that one can doubt all one's sense experiences, but that one cannot doubt one's own existence as a thinking being: 'cogito, ergo sum'
(I think, therefore I am). From this basis he argued that God must exist and cannot be a deceiver; therefore, his beliefs based on ordinary sense experience are correct.

His deductive reasoning from axioms and self-evident first principles is known as Descartes cogito, and the process by which it is reached is called 'Cartesian doubt'.

He maintained that all nature is subject to the laws of cause and effect and that all observable phenomena can be reduced to quantifiable elements.

In his 'Discourse on the Method for Rightly Conducting One's Reason and Searching for Truth in the Sciences' he laid the foundations for analytical geometry.