Feynman said it in “Cargo Cult Science,” Caltech’s 1974 commencement address, printed that month in the institute’s magazine Engineering and Science ↗ — volume 37, number 7 (June 1974), pages 10–13. The sentence appears there verbatim, and the address was later adapted as the closing chapter of Surely You’re Joking, Mr. Feynman! (1985). This is one of the rare famous-scientist quotations that survives a look at the primary document intact.
“The first principle is that you must not fool yourself — and you are the easiest person to fool” The first principle is that you must not fool yourself—and you are the easiest person to fool. —Richard P. Feynman, "Cargo Cult Science," Caltech commencement address, printed in Engineering and Science 37, no. 7 (June 1974), 10–13.
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Provenance
Where it comes from
Work
Cargo Cult Science
Occasion
Caltech commencement address, 1974
First published
Engineering and Science, vol. 37, no. 7 (June 1974), pp. 10–13
Reprinted in
Surely You’re Joking, Mr. Feynman! (W. W. Norton, 1985) — final chapter; p. 343 per Wikiquote
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.
From the passage on scientific integrity
The popular version stops at the word “fool,” which reverses the emphasis. Feynman’s point is that self-deception is the hard problem and honesty toward colleagues the easy one that follows from solving it. In the Caltech text the two clauses are joined by an em dash, not by “and,” and the very next thing he adds is that a scientist should not fool the layman either.
The Caltech Library’s digitized text of “Cargo Cult Science” ↗ contains the sentence verbatim, in the passage on scientific integrity, and is headed “Caltech’s 1974 commencement address.”
Caltech’s 1974 index of Engineering and Science ↗ supplies the full citation: Feynman, Richard P. (1974) Cargo Cult Science, Engineering and Science, 37 (7), pp. 10–13.
Wikiquote ↗ lists the line among its sourced Feynman quotations — not under “Disputed and/or attributed” — citing Cargo Cult Science and Surely You’re Joking, Mr. Feynman!, p. 343.
This is a 1974 publication — more than four decades after the 1931 line that separates public-domain from protected work in the United States — so the text of “Cargo Cult Science” remains under copyright, as does the 1985 W. W. Norton book that reprints it. The passage is quoted here in brief for identification, attribution and commentary; credit belongs to Richard P. Feynman and his publishers, and Caltech makes the full article freely readable on its own library site.
Rights status is our research, not legal advice — and copyright varies by country and edition. For commercial or published use, confirm with the rightsholder. See terms.
Fact-check
The name is right — the sourcing usually isn’t
This is not a fake Feynman quotation, and that is worth saying plainly. But it circulates almost entirely without its source, and the version that circulates has been quietly trimmed and de-punctuated along the way.
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The sourceQuote aggregators Unsourced
BrainyQuote’s entry ↗ reproduces the line with nothing but Feynman’s dates and the word “physicist” — no speech, no date, no publication, no page. That pattern is what makes an authentic quotation indistinguishable from an invented one at a glance.
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The wordingThe flattened one-liner Mis-worded
Caltech’s text joins the two halves with an em dash: “you must not fool yourself—and you are the easiest person to fool” ↗. The aggregator version replaces the dash with a plain “and,” and everyone stops at “fool,” cutting the sentences that give the principle its point — that not fooling other scientists is the easy half.
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The publicationSurely You’re Joking, Mr. Feynman! (1985) Later reprint
The meaningBusiness and self-help usage Recontextualized
The sentence is now quoted as generic advice about personal bias. In the original it is a rule of professional disclosure aimed at researchers, delivered inside an argument about pseudoscience and scientific integrity ↗ — the magazine’s own subtitle calls it “some remarks on science, pseudoscience, and learning how to not fool yourself.”
Feynman was addressing Caltech’s graduating class, and the subject was not motivation — it was fraud. His theme was what he called cargo cult science: research that reproduces the outward form of the scientific method while missing the substance, the way Pacific islanders after the war built runways and wooden headsets and waited for the planes to land. The magazine billed the piece as “some remarks on science, pseudoscience, and learning how to not fool yourself.”
The principle he set against it was not cleverness but disclosure. A scientist, he argued, owes an audience everything that might invalidate the result — the alternative explanations, the possibilities ruled out, the facts that cut against the theory as well as for it. He called this “a kind of leaning over backwards,” and the famous sentence is its foundation: fooling other people is the easy problem, because it only becomes possible once you have already succeeded at fooling yourself.
After you’ve not fooled yourself, it’s easy not to fool other scientists.
How the line escaped the lecture hall
The line is now quoted almost exclusively as a piece of general self-help — a caution about bias, ego or wishful thinking in business and in life. That reading is not wrong, but it is narrower than the original. Feynman’s next move in the address was to extend the duty outward: having dealt with self-deception and with one’s colleagues, he added a further obligation that he admitted was “not essential to the science” — that a scientist speaking publicly should not fool the layman either.
The address had a long afterlife. Caltech printed it in Engineering and Science in June 1974, and it was later adapted as the closing chapter of the 1985 memoir Surely You’re Joking, Mr. Feynman!, the book that carried Feynman to a mass readership — which is why most people today meet the sentence in a paperback rather than in a college magazine.
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More provenance-checked quotes on the same theme, each with its reuse status.
1918–1988 · American theoretical physicist · Nobel Prize in Physics, 1965
Richard Phillips Feynman (1918–1988) was an American theoretical physicist who shared the 1965 Nobel Prize in Physics for his work on quantum electrodynamics. He studied at MIT and Princeton, taught at Cornell, and spent the rest of his career at Caltech, where he became as famous for his teaching and his popular books — The Feynman Lectures on Physics, Surely You’re Joking, Mr. Feynman! — as for his physics. He later served on the Rogers Commission investigating the Space Shuttle Challenger disaster, an episode that made his insistence on not fooling yourself into public property.
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Cite this
Primary source
Feynman, Richard P. “Cargo Cult Science.” Engineering and Science 37, no. 7 (June 1974): 10–13. Caltech’s 1974 commencement address; adapted as the final chapter of Surely You’re Joking, Mr. Feynman!: Adventures of a Curious Character (New York: W. W. Norton, 1985).
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