Who said, "I think, therefore I am?"

"Cogito ergo sum." This philosophical proposition was first advanced by Rene Descartes, a 17th century French philosopher. The Latin phrase translates as "I think, therefore I am."

Originally written in his native French in his book "Discourse on the Method," he later translated it to Latin so as to be accessible to a larger audience and published this idea in his later work "Principles of Philosophy."

Although a plethora of volumes have been written on this idea and what exactly Descartes meant, in brief, he was proposing the idea that a human who thinks, questions reality and therefore must be self aware. The quote, placed in context of his writings addresses the idea that radical doubt, which he believed was the beginning of all productive thinking, distinguished fantasy, delusion and deception from reality or actual existence. So, for him, a person who doubts reality must think about it in more honest terms and therefore proves his or her existence in the world of reality. He used this idea to support his thesis that while emotion and senses might be deceptive and provide faulty or even inherently wrong information, the thinking mind connected humans to reality in a unique way.

This proposition has become a fundamental element of Western philosophy and has played an integral part in the development of philosophical thought ad argument for the past nearly 400 years.