What is “Fun?”“I’ll know it when I see it.”In 1964, in Jacobellis v. Ohio, the US Supreme Court needed to decide whether the state of Ohio could ban a film it called “obscene”—a concept people understood but were hard-pressed to define. Justice Potter Stewart, in his concurring opinion, wrote: “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. But I know it when I see it, and the motion picture in...