Justice Potter Stewart, in his concurrence to the majority opinion, created the standard whereby all speech is protected except for "hard-core pornography". As for what, exactly, constitutes hard-core pornography, Stewart said "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 involved in this case is not that." The film in question was Louis Malle's The Lovers.