[mycroft]

and by 21st century, we mean 1964 (to non-USians, though, credit is given for "why should we care?")[1]:

Wikipaedia:

Quote

In the obscenity case of Jacobellis v. Ohio (1964), [U.S. Supreme Court Justice Potter] Stewart wrote in his short concurrence that "hard-core pornography" was hard to define, but that "I know it when I see it, and the motion picture involved in this case is not that."

1.

Spoiler

This phrasing, because of this case, is about as famous and well-referenced in North America as "well, he would, wouldn't he?" is in the UK (and likely about as well- (or ill-)known on the other side of the Pond).