Richard Feynman in Cal Tech commencement:

After you have not fooled yourself, it’s easy not to fool other scientists. But not fooling yourself is far from easy. Because, liking your own ideas, you are the easiest person to fool.”

Richard Phillips Feynman, (born May 11, 1918, New York, New York, U.S.—died February 15, 1988, Los Angeles, California), American theoretical physicist who was widely regarded as the most brilliant, influential, and iconoclastic figure in his field in the post-World War II era.