René Descartes (1596-1650) was a French philosopher, mathematician, physicist, and writer. He has been called the "Father of Modern Philosophy," his influence in mathematics is also well known (as he is also known as the Father of Analytical Geometry.) Descartes was one of the key figures in the Scientific Revolution. He is, of course, best known for his celebrated reflections on what there might be which he can know with certainty, once he has rejected all he has been taught and all sense experience, and for his first important conclusion: "I think; therefore, I am." Hence, while he is capable of doubting or being deceived about nature around him (including his own body), he has something within him, the "I", which he identifies with the soul, the existence of which is for him absolutely certain, because, even in the process of doubting everything, he cannot deny that he is thinking.
Descartes published
Discourse on Method in 1637 as part of a work containing sections on optics, geometry, and meteorology. The fourth section, the Discourse, outlined the basis for a new method of investigating knowledge. He later (in 1641) published a more detailed exploration of the philosophical basis for this new approach to knowledge in Meditations on First Philosophy.