John Maynard Keynes is often quoted as saying: “When the facts change, I change my mind. What do you do, sir?” Quote Investigator could trace no source for it. The earliest attribution to him it located came from the economist Paul Samuelson, quoted in the Wall Street Journal in 1978.
Keynes did express a similar idea. In a 1924 essay on investment policy for insurance companies, he warned against an investor who “refuses to change his opinion merely because facts and circumstances have changed.”