As part of his concurring opinion in Jacobellis v. Ohio -- ruling that the French film "Les Amants" ("The Lovers") was not obscene and thus the state of Ohio could not ban its showing or prosecute a theater owner for having done so -- U.S. Supreme Court Justice Potter Stewart wrote what would become a well-known phrase, if not a strict legal definition.
I have reached the conclusion ... that under the First and Fourteenth Amendments, criminal laws in this area are constitutionally limited to hard-core pornography. 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.
* Summary (from ACLU of Ohio): @
* Transcript of ruling (from Legal Information Institute, Cornell University): @
* Oral arguments (from www.oyez.org): @
* "Fifty Years of "I know it when I see it' " (from www.concurringopinions.com): @
* "On 'I Know It When I See It' " (Paul Gewirtz, Yale Law Journal, 1996): @
* "Movie Day at the Supreme Court or 'I Know It When I See It': A History of the Definition of Pornography" (from corporate.findlaw.com): @