When J.P. Morgan Chase’s JPM -9.28% chief executive, James Dimon, disclosed a $2 billion trading loss during a hastily organized conference call on Thursday, he said: “This trading may not violate the Volcker rule, but it violates the Dimon principle.”

Mr. Dimon didn’t say what the Dimon principle is, and a spokesman for the nation’s largest bank by assets didn’t respond to requests for comment.

The Feynman principle, however, is simple: “You must not fool yourself—and you are the easiest person to fool,” as the Nobel Prize-winning physicist Richard Feynman put it.

Asked on April 13 whether J.P. Morgan’s trading operation posed significant risks to the bank, Mr. Dimon called it a “tempest in a teapot.” The bank’s chief financial officer insisted the London-based division was merely “protecting that balance sheet,” adding that J.P. Morgan was “very comfortable with the positions we have.”

The moguls of J.P. Morgan, in letting a complex risk run wild and denying any potential for error until it was too late, are a reminder that one of the biggest dangers in finance is self-deception.

For investors, the bigger the commitment, the more certain they become that they must have been right to make it—and the harder it becomes to let go.

The literal meaning of the word “invest”—from the Latin vestire, to clothe or dress—is to wrap oneself up in something. Experiments at racetracks and elsewhere have shown that people who bet on an outcome become up to three times more confident that it will occur than people who didn’t put up any money.