Rationalism
René Descartes portrait by Frans Hals (1649)
About the same time when Francis Bacon in England came up with the idea that the careful study of nature and inductive logic is the best way to learn about the real world, another key figure of the scientific revolution, the french mathematician and philosopher René Descartes (1596-1645), built an entirely different and, in some ways, opposite system of thought, where instead of nature, the human mind was chosen as the origin of all true knowledge. The axiom on which Cartesian philosophy is built is “thought exists”, from which follows the famous “cogito ergo sum” (I think, therefore I exist). He discarded sensory perception as unreliable and built a system of thought based on deduction only, subject to rigorous tests of doubt and methodological skepticism, making sure every statement is absolutely flawless. Doing that resulted a number of great advances in the fields of mathematics and logic and sparkled a centuries-long argument between the Rationalists and the Empiricists, which, although partially reconciled, sparks between philosophers even today.
The work of Descartes was continued by Gottfried Leibniz (1646-1716) who ventured further into the world if the mind, giving it atoms he called Monads, and came up with the early concept of Anthropic Principle and by Baruch Spinoza (1632-1677) who in fact critisized the whole idea that the mind is different from the external reality, and postulated that all of Nature, both physical and the mental worlds are one and the same. Unlike the two other Rationalists, who fully supported monotheistic religion, Spinoza applied reason to religions and found that they held no grounds. The last of the rationalists was Immanuel Kant (1724–1804), who ended the original debate with Empiricism, having built the famous philosophical system on the postulate that pure experience is always subjective but pure reason can only lead to illusions.
Despite their disregard of experimental evidence, the rationalists have improved science by forever merging it with logic and mathematics. Scientists still rely on rigorous proofs and detailed deductions in their everyday work.