Jim Holt on how philosophers have explained our sense of humour

How many kinds of joke are there? There are classic jokes. ("Who was that lady I saw you with last night?" "That was no lady, that was my wife.") There are political jokes, such as Ronald Reagan's definition of liberalism: "If it moves, tax it. If it keeps moving, regulate it. If it stops moving, subsidise it." The Iraq war has spawned an entire new category of neocon jokes: "How many neocons does it take to screw in a light bulb? None - President Bush has announced that in three months the light bulb will be able to change itself."

There are nice jokes that can be told in any drawing room. ("What does a snail say when riding on the back of a turtle?" "Whee!") And there are naughty jokes, such as the one about the woman who flies into Boston eager to enjoy a plate of the fish for which that city is famous. "Where can I get scrod?" she asks the driver as she gets into the cab. "Gee," he replies, "I've never heard it put in the pluperfect subjective before." Or the one about the successful diet Bill Clinton went on: "He's lost so much weight, now he can see his intern." And there are jokes that are inadvertent as well as jokes that are deliberate - and some that are, paradoxically, both at the same time, such as the London newspaper headline during the second world war: "British Push Bottles Up Germans".

Could any theory make sense of even this small sampling? There are three competing traditions, all a bit mouldy, that purport to explain how humour works. The "superiority theory" - propounded in various forms by Plato, Hobbes and Bergson - locates the essence of humour in the "sudden glory" (Hobbes) we feel when, say, we see Bill Gates get hit in the face with a custard pie. According to this theory, all humour is at root mockery and derision, all laughter a slightly spiritualised snarl.

The "incongruity theory", held by Pascal, Kant and Schopenhauer, says that humour arises when the decorous and logical abruptly dissolves into the low and absurd. "Do you believe in clubs for small children?" WC Fields is asked. "Only when kindness fails," he replies.

Why either of these perceptions - superiority or incongruity - should call forth a bout of cackling and chest heaving remains far from obvious. It is an advantage of the third theory, the "relief theory", that it at least tries to explain the causal link between humour and laughter. In Freud's version, the laughable - ideally a naughty joke - liberates the laughter from inhibitions about forbidden thoughts and feelings. The result is a discharge of nervous energy - a noisy outburst that, not incidentally, serves to distract the inner censor from what is going on.....