Can you provide a famous quote by Descartes and explain its significance in his philosophy?

One famous quote by Descartes is "Cogito, ergo sum" which means "I think, therefore I am." This quote signifies Descartes' emphasis on the importance of doubt and skepticism in the pursuit of knowledge. It highlights the idea that the act of thinking proves one's existence, as the very act of doubting or thinking about one's existence implies that one must exist in some form. This quote is a foundational concept in Descartes' philosophy, as it serves as the starting point for his method of doubt and his quest for certain knowledge.