An astronomer, a physicist and a mathematician are on a train in Scotland. The astronomer looks out of the window, sees a black sheep standing in a field, and remarks, "How odd. Scottish sheep are black." "No, no, no!" says the physicist. "Only some Scottish sheep are black." The mathematician rolls his eyes at his companions' muddled thinking and says, "In Scotland, there is at least one sheep, at least one side of which appears to be black from here."
(This version is from Wikipedia.)
Previously approximated here.
I still habitually complete this joke with:
"Actually," says the stage magician, "we merely know that there exists something in Scotland which appears to be a sheep which is black on at least one side when viewed from this spot."
Though I'm now tempted to add:
"Hmph," snorts the cognitive psychologist. "Such presumption. An event occurred that we experienced as the perception of a black sheep, only one side of which was visible, standing on what we believed to be a field in Scotland."
Here's the new thread for posting quotes, with the usual rules: