Descartes’ most famous statement is Cogito ergo sum, “I think, therefore I exist.” With this argument, Descartes proposes that the very act of thinking offers a proof of individual human existence. Because thoughts must have a source, there must be an “I” that exists to do the thinking. In arguments that follow from this premise, Descartes points out that although he can be sure of nothing else about his existence—he can’t prove beyond a doubt that he has hands or hair or a body—he is certain that he has thoughts and the ability to use reason.

Descartes asserts that these facts come to him as “clear and distinct perceptions.” He argues that anything that can be observed through clear and distinct perceptions is part of the essence of what is observed. Thought and reason, because they are clearly perceived, must be the essence of humanity. Consequently, Descartes asserts that a human would still be a human without hands or hair or a face. He also asserts that other things that are not human may have hair, hands, or faces, but a human would not be a human without reason, and only humans possess the ability to reason.