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Who really said it

Anyone who claims to understand quantum physics is lying or crazy

Nobody on record — Feynman said something else Unknown Anonymous paraphrase · circulating as ‘legend’ by the 2010s
Disputed In copyright

This is a right-person, wrong-words case. No lecture, book, letter, interview or recording of Feynman contains this sentence — and the popular-science outlets that use it hedge it themselves, introducing it with the phrase ‘according to legend’. What he really wrote is one line away and considerably better: ‘I think I can safely say that nobody understands quantum mechanics’, in The Character of Physical Law (1965), chapter 6, page 129 — drawn from his 1964 Messenger Lectures at Cornell, filmed by the BBC.

Last verified 21 Jul 2026 · how we verify · spot an error?

For presentations

Put it on a slide

Disputed credit — copy with the correction

“Anyone who claims to understand quantum physics is lying or crazy” Feynman actually said: "I think I can safely say that nobody understands quantum mechanics." The Character of Physical Law (1965), ch. 6, p. 129. The "lying or crazy" version has no source.

The slide-ready mistake: crediting this to Richard Feynman. Use the credit shown above instead.

Can you use it?

Still under copyright. A short, credited quote is generally fine for talks, teaching, and internal decks; for commercial, published, or large-scale reuse, get permission.

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In context

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Provenance

The source

Genuine line
‘I think I can safely say that nobody understands quantum mechanics’
Document
The Character of Physical Law (1965), ch. 6, p. 129
Delivered
Messenger Lectures, Cornell University, 1964; filmed by the BBC
Source for ‘lying or crazy’
None located

There might have been a time when only one man did, because he was the only guy who caught on, before he wrote his paper. But after people read the paper a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I think I can safely say that nobody understands quantum mechanics. The Character of Physical Law, 1965

The genuine passage, with the sentence in the position that gives it its force. He is comparing quantum mechanics to relativity, which he says plenty of people did understand once they read the paper. The joke lands because of the contrast, and because of the mock-cautious “I think I can safely say” — the hedging of a man who knows exactly how large a claim he is making.

Wikiquote — the sourced Feynman line, with page citation

How we traced it

  • The genuine sentence is easy to pin down: chapter 6 of The Character of Physical Law, ‘Probability and Uncertainty — the Quantum Mechanical View of Nature’, page 129. The book is the transcript of lectures he gave at Cornell in 1964, and the BBC film of them still exists, so this is one of the rare quotations you can both read and hear the citation ↗.
  • The ‘lying or crazy’ version has no equivalent anchor. It appears in newspaper and popular-science writing — a widely syndicated 2013 report on a survey of physicists opens with it — but as a piece of received wisdom, introduced with hedges like ‘according to legend’ rather than a citation an example ↗.
  • One honest limit on this page: proving a negative in a corpus as large as Feynman’s — lectures, books, recordings, remembered conversation — is not possible. What we can say is that no one who quotes the sentence produces a source for it, and that the reference works which do carry citations for his quantum remarks carry the 1965 wording instead.
In copyright

Feynman’s genuine sentence is in copyright. He died in 1988, so in the UK and EU the text is protected until the end of 2058, and The Character of Physical Law — first published 1965 and continuously in print — is protected in the United States as well, with rights administered through his publishers and estate. Short quotation with attribution is normal practice; reproducing the passage at length is not. The ‘lying or crazy’ version has no identified author, so no term can be calculated for it, which is a gap in the record rather than a licence. This is not legal advice.

Fact-check

The attribution problem

Nothing was fabricated out of nothing here. A real remark by a real physicist got sharpened, twice, until it said something he did not say and would not have said.

Right man, wrong words no source located

‘Lying or crazy’ appears nowhere in Feynman’s lectures, books, letters or recordings. Even the outlets that print it flag it as legend rather than citing it an example ↗.

A confession became an accusation meaning drift

‘Nobody understands quantum mechanics’ is Feynman including himself — a statement about the limits of intuition, delivered by someone who had just won a Nobel Prize for working in the field anyway. ‘Anyone who claims to understand it is lying or crazy’ points at other people and calls them frauds. Same subject, opposite posture.

Also widely credited, also unsourced second-order paraphrase

This tidier epigram is the most common version online. It has no located source either — it is the 1965 sentence rewritten into a paradox, which is the step that made the ‘lying or crazy’ escalation possible.

A different real line, often conflated conflation

Bohr is associated with ‘anyone who is not shocked by quantum theory has not understood it’. It is a separate saying with its own attribution questions, and it is not the source of this one.

The words are not Feynman’s and have no source. His genuine line is ‘I think I can safely say that nobody understands quantum mechanics’ — The Character of Physical Law (1965), chapter 6, page 129 the citation ↗.

Context

Why it resonates

Quantum mechanics is the one branch of physics whose predictions are flawless and whose meaning is still argued about, so a licence to stop trying to picture it comes as enormous relief. That is what the line is doing for most people who post it.

The fake version adds something the real one refuses: permission to sneer. Feynman’s point was that the theory defeats intuition, including his own. The rewrite turns that into a charge of dishonesty against anyone who says otherwise, which is both less generous and less interesting.

He said nobody understands it, and included himself. The rewrite points at other people.

The genuine remarks, in order

1964: Feynman delivers the Messenger Lectures at Cornell; the BBC films them.

1965: they appear as The Character of Physical Law. Chapter 6, page 129 — ‘I think I can safely say that nobody understands quantum mechanics.’

1982: in ‘Simulating Physics with Computers’ he says we always have a great deal of difficulty understanding the world view quantum mechanics represents — and adds that he is not sure there is a real problem.

1985: in QED he tells readers not to turn away because they do not understand it, on the grounds that his own physics students do not understand it either.

The through-line is modesty about intuition, not contempt for other physicists.

Explore

Filed under

More provenance-checked quotes on the same theme, each with its reuse status.

Knowledge Humility

Go further

Dig deeper

Trusted places to keep pulling the thread — primary archives, fact-checks, and scholarship. We show how we used each one before you leave.

Richard Feynman — sourced quotationsen.wikiquote.org

The genuine line with its full citation: The Character of Physical Law (1965), ch. 6, p. 129.
The Character of Physical Lawen.wikipedia.org

Background on the 1964 Messenger Lectures at Cornell, the BBC filming, and the 1965 book.
Physicists Disagree Over Meaning of Quantum Mechanics, Poll Showslivescience.com

A widely syndicated 2013 article that opens with the ‘lying or crazy’ version — and hedges it as legend rather than citing it.
Talk:Richard Feynman — sourcing discussionen.wikiquote.org

Editors working through which Feynman quantum lines have documents behind them and which do not.

Popularly credited author

Why it gets pinned on Feynman

Richard Feynman

1918–1988 · theoretical physicist

Richard Feynman (1918–1988) shared the 1965 Nobel Prize in Physics for quantum electrodynamics and was, by some distance, the most quotable physicist of his century — a bongo-playing, safe-cracking lecturer who talked about his own subject with cheerful irreverence. That reputation is why this sentence sticks to him: it sounds like the sort of thing he would say. He did say the sort of thing he would say. He just said it more precisely, and with better manners.

For writers & researchers

Cite this

Primary source — Chicago

Feynman, Richard P. The Character of Physical Law (1965), ch. 6, ‘Probability and Uncertainty — the Quantum Mechanical View of Nature’, p. 129. Delivered as the Messenger Lectures, Cornell University, 1964. No source exists for the ‘lying or crazy’ wording.

MLA (9th ed.)

Feynman, Richard P. The Character of Physical Law. 1965, p. 129.

APA (7th ed.)

Feynman, R. P. (1965). The Character of Physical Law (p. 129).

This verification page

Quotle.info. “Who really said ‘Anyone who claims to understand quantum physics is lying or crazy’?” Last verified 21 Jul 2026. https://quotle.info/who-said/anyone-who-claims-to-understand-quantum-physics-is

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