A comment on the Miers/Stewart/Hewitt thread -- "Potter Stewart is most famous for recognizing obscenity when he sees it" -- reminded me of one of my favorite facts. It's true that this quote is indeed what Stewart is famous for, but people generally miss the follow-up to the quote: Nine years later Justice Stewart joined the dissent in Miller v. California, and would have thus held that pornography 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.
Stewart's willingness to change his mind, and his ultimate preference for a clear rule of protecting speech -- including sexually themed speech of the sort that he probably found personally quite distasteful -- strikes me as much more important than his original endorsement of a more vague, case-by-case approach.