“A joke's a very serious thing.”

The Abolition of Work (1985)
Context: These experts who offer to do our thinking for us rarely share their conclusions about work, for all its saliency in the lives of all of us. Among themselves they quibble over the details. Unions and management agree that we ought to sell the time of our lives in exchange for survival, although they haggle over the price. Marxists think we should be bossed by bureaucrats. Libertarians think we should be bossed by businessmen. Feminists don't care which form bossing takes so long as the bosses are women. Clearly these ideology-mongers have serious differences over how to divvy up the spoils of power. Just as clearly, none of them have any objection to power as such and all of them want to keep us working.
You may be wondering if I'm joking or serious. I'm joking and serious. To be ludic is not to be ludicrous. Play doesn't have to be frivolous, although frivolity isn't triviality: very often we ought to take frivolity seriously. I'd like life to be a game — but a game with high stakes. I want to play for keeps.

Source: Esther: A Novel (1884), Ch. VII

Hemingway is describing his friend, the famous bullfighter Antonio Ordóñez.
Source: The Dangerous Summer (1985), Ch. 3

As quoted in "A View from the Asylum" in Philosophical Investigations from the Sanctity of the Press (2004), by Henry Dribble, p. 87
Attributed from posthumous publications

Sometimes attributed to Heisenberg, this was actually a statement made by Niels Bohr, as quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24
Some things are so serious that one can only joke about them.
Variant without any citation as to author in Denial is not a river in Egypt (1998) by Sandi Bachom, p. 85
Misattributed

As quoted in The Genius of Science: A Portrait Gallery (2000) by Abraham Pais, p. 24
Some things are so serious that one can only joke about them.
Variant without any citation as to author in Denial is not a river in Egypt (1998) by Sandi Bachom, p. 85.

Source: Invitation to Sociology (1963), p. 11