Feynman identifies self-deception as a central risk in scientific reasoning. People can become attached to an explanation, result or expectation and then interpret evidence in ways that support what they already want to believe. His warning is that avoiding this tendency requires deliberate care, because the person conducting the inquiry is also capable of misleading themselves.

In the wider passage, Feynman connects this principle to scientific honesty. Before communicating results responsibly to others, a researcher must first examine whether their own assumptions, methods and interpretations are sound. The quotation therefore goes beyond deliberate dishonesty and focuses on unconscious bias, selective attention and wishful thinking. Its broader application is to any form of reasoning in which personal interests or prior beliefs may influence judgement. The principle is to test one’s own conclusions as critically as those of other people.

Full quotation

“The first principle is that you must not fool yourself—and you are the easiest person to fool. So you have to be very careful about that. After you’ve not fooled yourself, it’s easy not to fool other scientists. You just have to be honest in a conventional way after that.”