The most-quoted line about changing your mind was never said by the man credited with it
Everyone attributes it to a famous economist. The paper trail points somewhere else — and it's a better story either way.
"When the facts change, I change my mind. What do you do, sir?" gets attached to John Maynard Keynes in speeches, op-eds, and probably several arguments you've personally lost. Researchers who've traced the line back through print have never found Keynes saying it. What they've found instead is close, but not that: a 1924 essay where he wrote that an investor who refuses to revise an opinion after the facts change is the one who eventually suffers for it — same idea, different words, no memorable rejoinder attached.
The line that actually matches almost word for word came from Paul Samuelson, the economist, on live television in 1970. Asked why his textbook kept revising its inflation forecasts downward, he said simply: "When events change, I change my mind. What do you do?" Eight years later, retelling the story, Samuelson himself credited it to Keynes — maybe misremembering, maybe being generous to a hero, maybe just liking how it sounded coming from someone more famous than he was.
Either way, a good line about intellectual honesty ended up being laundered through exactly the kind of unreliable memory it was warning against. Nobody set out to fabricate a quote. Someone just liked an idea enough to want it to have a better author.
Which is the real lesson hiding under the fake one: changing your mind under new evidence is rare enough that we'll apparently even borrow a name to make the habit sound more credible than it is.