"I suppose therefore that all I see is false. I believe that none of what my deceptive memory represents to me ever existed."

"The existence of God is as certain as any geometrical demonstration."

"I concluded that I was a substance whose whole essence or nature consists in thinking, and whose being does not depend on any place or any material thing."

"I was very surprised to find that I had been mistaken more often than not in my life."

"It is proper to a philosopher to doubt everything, at least once."