What did René Descartes research?

René Descartes was a French philosopher, mathematician, and scientist known for his contributions to philosophy and mathematics. He is best known for his work on Cartesian coordinate systems in geometry and his philosophical method of doubt, famously encapsulated in the phrase "Cogito, ergo sum" ("I think, therefore I am"). Descartes also explored the nature of existence, the mind-body relationship, and the principles of rationalism, laying foundational ideas for modern Western philosophy. His works, including "Meditations on First Philosophy" and "Discourse on the Method," significantly influenced various fields, including mathematics, physics, and epistemology.