‘Twas the saying of an ancient Sage, “That Humour was the only Test of Gravity, and Gravity of Humour. For a Subject which would not bear Raillery is suspicious; and a Jest which would not bear a serious Examination is certainly false Wit.”

Anthony Cooper, 3rd Earl of Shaftesbury (1671-1713) English politician and philosopher
Sensus Communis: An Essay on the Freedom of Wit and Humour, Part 1, Sec. 5 (1709)
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Often incorrectly attributed to Aristotle. Shaftesbury, according to his footnote, is paraphrasing from Aristotle quoting Gorgias Leontinus. The Latin translation is "Seria risu, risum seriis discutere" ("In arguing one should meet serious pleading with humor, and humor with serious pleading"). Shaftesbury's second sentence is his own commentary.

In Lord Chesterfield, in a letter to his son (6 Feb 1752), rendered it, "Ridicule is the best test of truth."