Fundamentally, Descartes’s contribution to psychology was to make the concept of mind front and center of his philosophy. His famous phrase Cogito ergo sum (I think therefore I am) links the mental function of thinking to the proof of his very existence. A naturalist who carefully observed the natural world and even dissected animals, he was extremely interested in the relationships between mental and bodily processes. In fact, Cartesian dualism, the notion that the mind and the body are separate entities, continues to inspire debate to this day.