René Descartes advocated for rationalism through his famous statement "I think, therefore I am," emphasizing the importance of reason as a source of knowledge. His approach to philosophy relied on systematic doubt, urging individuals to question accepted beliefs and rely on clear and distinct perceptions. Descartes' method set the foundation for modern philosophy and scientific inquiry, promoting the idea that human thought and reason could lead to truth. By valuing skepticism and analysis, Descartes influenced numerous fields, including mathematics and the natural sciences, encouraging a shift toward rational thought in the pursuit of knowledge.