In the landmark obscenity case Jacobellis v. Ohio, Supreme Court Justice Potter Stewart famously refused to define which level of “hard-core pornography” passed the point of constitutional protection.

“I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so,” Stewart wrote. “But I know it when I see it, and the motion picture involved in this case is not that.”