“You must not fool yourself”: did Feynman really say it?

What Feynman said

“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.”

Key findings

The quote is genuine: it appears in “Cargo Cult Science”, Feynman’s commencement address at Caltech in 1974.

The printed version is in Caltech’s magazine Engineering and Science, June 1974, on page 12.

In context it’s about scientific integrity: reporting everything that could show your result is wrong, which Feynman called “a kind of leaning over backwards”.

His example was the history of measurements of the electron’s charge after Robert Millikan, which crept up slowly as people hesitated to report numbers far from his.

Where it comes from

In 1974 Feynman gave the commencement address at Caltech. Caltech’s magazine Engineering and Science printed it in its June 1974 issue as “Cargo Cult Science”, with the subtitle “Some remarks on science, pseudoscience, and learning how to not fool yourself.” The famous line sits on page 12 of that issue.

The title comes from his central image. He describes islanders in the South Seas who, after seeing planes land with supplies during the war, build runways, light fires along them and put a man in a wooden hut with “two wooden pieces on his head like headphones”. Everything looks right. “But it doesn’t work. No airplanes land.” Research that copies the outward form of science without its substance, he says, is the same thing.

What he meant

The missing ingredient, in his words, is “a kind of utter honesty—a kind of leaning over backwards”. If you do an experiment, report everything that might make it invalid, not only what supports your idea.

His example is a little embarrassing for physics. After Robert Millikan measured the charge of the electron with falling oil drops, later measurements came out “a little bit bigger” each time, until they settled on a higher value. Feynman’s explanation: when people got a number too far from Millikan’s, they looked for a reason it might be wrong, and when they got one close to his, they didn’t look so hard.

Then comes the line: you must not fool yourself, and you are the easiest person to fool. The next sentences matter too: “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.”

Our take

This one is the real thing, which is rarer than you’d think for a famous Feynman quote. When it’s shared as a general piece of life advice, it keeps its spirit. But the original is sharper and more practical: it’s a rule about what you owe the people who will rely on your results. Fooling yourself isn’t a moral failing in his telling, just the default, which is why it takes deliberate effort to avoid.

It’s also the line this whole site is built on: the things we’re most sure of are the ones we check least.

How we check a quote

We look for the original: the book and page, the lecture, the letter, the recording or the report. Real means we found the words in Feynman’s own work. Paraphrased means he said something close, but not those words. Misattributed means someone else said it, or it first appears somewhere that isn’t Feynman. No reliable source found means we couldn’t trace it to any original. Quotations are kept short and come with our own commentary. If you have a source we missed, write to us via the imprint and we’ll update the page.

“You must not fool yourself”: did Feynman really say it?

Verdict: Real