What was Justice Potter Stewart referring to when he said,"I know it when I see it"?

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Justice Potter Stewart was referring to the definition of obscenity in the case of Jacobellis v. Ohio (1964). When asked to define obscenity, he famously stated, "I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description [of hard-core pornography]; 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."

What Was Justice Potter Stewart Referring to When He Said,"I