René Descartes is the ancient philosopher known for the famous quote, "I think, therefore I am." This statement encapsulates his philosophical stance on existence and consciousness, positing that the very act of thinking is proof of one's existence. Descartes, often referred to as the father of modern philosophy, emphasized reason as the primary source of knowledge and explored the relationship between the mind and body. His work laid the groundwork for subsequent philosophical inquiry and significantly influenced the development of modern science and rationalist thought.