In 1790 the German philosopher Immanuel Kant in The Critique of Judgment focused on the requirement of surprise when he claimed that laughter is an emotion that arises from a strained expectation suddenly reduced to nothing. William Hazlitt, in his 1819 Lectures on the Comic Writers, credited laughter as coming from the incongruity that results when one idea disconnects or is bumped up against another feeling. Arthur Schopenhauer agreed in 1844, when he explained in The World as Will and Idea that laughter is a way of acknowledging an incongruity between the conceptions that listeners or viewers hold in their minds and what happens to upset their expectations.
The incongruity theory is especially powerful in explaining humor across different genres, including accidental humor and humor in nature. Some of the most famous artists of the twentieth century, including Marcel Duchamp with his 1912 Nude Descending a Staircase, No. 2, surprised and amused (and sometimes offended) the public by breaking with the expectation that an artist's job was to faithfully re-create items as seen by the human eye. Playful dance companies and playful musicians startle audiences by suddenly changing their patterns, as did Franz Joseph Haydn (1732–1809) in his Symphony no. 94, also known as the Surprise Symphony. Haydn interspersed fortissimo chords into soft repetitive sequences to wake up slumbering audiences. Designers of theme-park hotels and of much of the community art that decorates modern American cities play with surprise and incongruity. Even comedians who tell stories in sets of three (two to establish a pattern and one to break it) are relying on surprise and incongruity.
Scatological humor is incongruous in that it "unmasks" people as it reminds them of their animal nature. This was one of the ideas expressed by Sigmund Freud in his 1905 Jokes and Their Relation to the Unconscious. Freud viewed the jokes that people told as a window to their minds. He thought that jokes were more likely to come from the id or subconscious, while people's other communications come from the superego, where they are refined by a public consciousness. Freud's work with jokes and his belief that humor is basically tendentious and hostile is not as respected as is his other work because modern critics, especially feminists, point out that the source of his jokes (his patients) were far from being a typical sample. Other critics reject the idea that jokes are formed in the subconscious in the same way that dreams are.
The one concept that the general public recognizes from Freud's work is that of the Freudian slip. These are verbal mistakes that people make, which Freud said revealed what they really wanted. In actual conversations, Freudian slips may or may not reveal inner desires. Sometimes they are simply pronunciation or spelling errors, as with the examples that Richard Lederer collected for his popular Anguished English (1987). On the other hand, when creative writers put Freudian slips into the mouths of their characters, their intention is to communicate something about the speaker's personality. Norman Lear was a master at this when in the popular television show All in the Family (1971–1979) he had Archie Bunker reveal his lack of education and his xenophobic tendencies with such phrases as "Blackberry Finn," "pushy imported ricans," "wel-fare incipients," and "the immaculate connection."