Richard Feynman on not fooling oneself:

Science is a way of trying not to fool yourself. The first principle is that you must not fool yourself, and you are the easiest person to fool.

Richard Feynman, “What is and What Should be the Role of Scientific Culture in Modern Society” (1964), Read before the Galileo Symposium (Cited by Koen Vervloesem in De Conceptuele Ingenieur)