In a well-known U.S. Supreme Court opinion on pornography, Associate Justice Potter Stewart wrote:
... criminal laws in this area are constitutionally limited to hard-core pornography. I shall not attempt 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.
This "know it when I see it" principle has some adherents in the financial community, particularly with regard to the ambiguities surrounding the question, "What is a bank?"