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."