Potter Stewart was the US Supreme Court justice who said, in the case Jacobellis v. Ohio (1964), “I shall not today attempt further to define (pornography)… But I know it when I see it…”

I was reading In Praise of Slowness by Carl Honoré, and he quotes Augustine: “What then is time? If no one asks me, I know: if I wish to explain it to one that asketh, I know not…” (Book XI, ~400AD)

So Mr. Stewart had a bit of prior art on that idea.