“If you can’t explain it to a freshman, you don’t understand it”: did Feynman say it?

Key findings

The popular rule comes from an anecdote told by Caltech physicist David Goodstein in Physics Today, February 1989, a year after Feynman’s death.

Goodstein had asked Feynman why spin-½ particles obey Fermi-Dirac statistics. Feynman promised a freshman lecture on it, then came back and said: “I couldn’t reduce it to the freshman level. That means we really don’t understand it.”

The “we” matters: Feynman was talking about physicists’ understanding of one deep result, not judging anyone else.

Goodstein calls the freshman level “the standard by which he measured whether something was really understood”. The versions with a six-year-old or “simple terms” have no Feynman source we could find.

Where it comes from

In February 1989, a year after Feynman died, Physics Today published a piece by Caltech physicist David Goodstein on Feynman as a teacher, “Richard P. Feynman, Teacher”. On its last page he explains why Feynman agreed to spend two years teaching beginners: partly for the audience, partly because he cared about students, and partly for “the sheer challenge of reformulating physics, as he understood it, so that it could be presented to young students.”

Then comes the story. Goodstein once asked Feynman to explain why spin-½ particles obey Fermi-Dirac statistics, a deep result of quantum physics. “Gauging his audience perfectly, he said, ‘I’ll prepare a freshman lecture on it.’ But a few days later he came to me and said: ‘You know, I couldn’t do it. I couldn’t reduce it to the freshman level. That means we really don’t understand it.’”

A story, not a slogan

Three things get lost when this becomes a poster.

First, it’s reported speech. These are Feynman’s words as Goodstein remembered them, which is good evidence but not a recording.

Second, Feynman said “we really don’t understand it”. He was talking about physics as a whole and about one particular deep result, and he included himself. The popular version is aimed at you.

Third, it was a standard Feynman held himself to. Goodstein calls explaining at the freshman level “the standard by which he measured whether something was really understood”. That’s a high bar for experts, not a test for catching people out.

The other versions

“If you can’t explain it to a six-year-old, you don’t understand it yourself” and “If you can’t explain it simply, you don’t understand it well enough” float around with Feynman’s name, and sometimes Einstein’s. We found no Feynman source for either, and the Wikiquote discussion traces the first-year-student version back to Goodstein’s anecdote too.

Our take

Paraphrased, and generalised. The real moment is better than the slogan: one of the best physicists of the century tried to explain something to beginners, couldn’t, and concluded that the problem lay with physics, not with the beginners. If you use it, quote Goodstein’s account and say it’s his.

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.

“If you can’t explain it to a freshman, you don’t understand it”: did Feynman say it?

Verdict: Paraphrased

What Feynman said, as Goodstein reported it