In the fall of 1989 a female student at Case Western Reserve University told me the following Descartes joke.
Our man stops at a bar, the 'tender asks whether he wants a drink, Descartes says, "I think not, then disappears. I replied, pedantically, "I think therefore I am" is not logically equivalent to "I think not therefore I am not" any more than "I am walking therefore I am moving" is logically equivalent to "I am not walking therefore I am not moving." So the joke rests on a logical mistake.
But this is true of many if not most jokes. I have toyed with the notion that most humor stems from logico-conceptual incoherence of one sort or another, ambiguity, amphiboly, equivocation and various formal mistakes. Another example is Yogi Berra's "If you come to a fork in the road, take it." Or: "Who was that lady I saw you with last night? That was no lady, that was my wife!" Or: "I see you got a haircut. No, I got 'em all cut."