What aspect of philosophy did René Descartes focus on?

René Descartes is best known for his emphasis on the role of deductive reasoning in philosophy, which is grounded in his method of systematic doubt and inquiry. He believed that through rational thought and clear and distinct ideas, one could arrive at indubitable truths. Descartes famously stated, "Cogito, ergo sum" (I think, therefore I am), which illustrates his focus on the act of thinking as a foundation for knowledge. His work laid the groundwork for modern philosophy and mathematics, influencing ideas about certainty, existence, and the nature of reality through a rational lens. While he certainly made strides in mathematics, particularly with Cartesian coordinates, his philosophical contributions primarily revolve around deduction as a pathway to knowledge and truth.