Supposedly, Aristotle said:
The secret to humor is surprise.
Why does Aristotle reckon that the secret to humor is surprise?
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The most likely sources of that Aristotle quote (if it is one) would be the Poetics and the Rhetoric. Searching the Poetics for "humo[r]", "secret" and "surprise" (individually) in the translations by Samuel Henry Butcher (1922) and Ingram Bywater (1898) turns up nothing that looks relevant. Then again, since The Name of the Rose every man and his dog knows that the second book of Aristotle's Poetics is lost.
What does Aristotle's Rhetoric say? In Book 3, chapter 11, we find the following passage:
And what Theodorus calls “novel expressions” arise when what follows is paradoxical, and, as he puts it, not in accordance with our previous expectation; just as humorists make use of slight changes in words. The same effect is produced by jokes that turn on a change of letter; for they are deceptive. These novelties occur in poetry as well as in prose; for instance, the following verse does not finish as the hearer expected:
“And he strode on, under his feet—chilblains,”
The above comes fairly close to the claim that "the secret to humor is surprise" but I have not found evidence that Aristotle used that terse formulation.
The earliest occurrence of this quotation that I can find is from 2004:
“The secret to humor is surprise” —Aristotle
The Bathroom Readers’ Institute, ed. (2004). Uncle John’s Colossal Collection of Quotable Quotes, p. 22. Oregon: Bathroom Readers’ Press.
Suffice it to say that the “Bathroom Readers’ Institute” is not a scholarly or reliable source, and either one of the contributors to this volume invented the quotation, or else they took it from some other, equally unreliable, source.
Aristotle actually says (roughly speaking) that the secret to tragedy is surprise:
But again, Tragedy is an imitation not only of a complete action, but of events terrible and pitiful. Such an effect is best produced when the events come on us by surprise; and the effect is heightened when, at the same time, they follow from one another.
Aristotle. Poetics, IX.11. Translated by S. H. Butcher (1898). The Poetics of Aristotle, p. 39. London: Macmillan.