Justice Potter Stewart is famous for having proposed the "I know it when I see it" test about what constitutes obscenity (i.e., constitutionally unprotected pornography). Some have characterized the endorsement test, or the Establishment Clause caselaw dealing with religious speech more broadly, as reflecting the same view.
Many people who know the quote, though, don't know the follow-up: Nine years later Justice Stewart joined the dissent in Miller v. California, and would have thus held that such material is categorically constitutionally protected (at least where no unwilling viewers or underage viewers are involved). And the dissent's reasoning focused largely on the vagueness of the existing tests for what's constitutionally protected and what's not.
So Stewart thought he knew it when he saw it. But after seeing enough cases, it seems that he either lost confidence in his own ability to know what should be protected, or concluded that such a test was in any event no way to run a legal system.