The philosopher known for the quote "I think, therefore I am" is René Descartes. This statement, originally "Cogito, ergo sum" in Latin, is fundamental to his philosophy and reflects his approach to existence and knowledge. Descartes proposed that the very act of doubting or questioning one’s existence serves as proof of the thinking self. His work in rationalism laid the groundwork for modern philosophy, emphasizing reason as the primary source of knowledge. Descartes is also noted for his contributions to mathematics, particularly in developing Cartesian coordinates, thus linking geometry and algebra, which have had lasting impacts on both disciplines.