Is it true or is it just slick -- I'm not quite sure but it crossed my mind:
Good people are like good pornography: As Supreme Court Justice Potter Stewart is said to have said approximately after conferring with a law clerk on a pornography case: "I may not know what it is, but I know it when I see it."
Are good people like that? I sort of think so, but I can't prove it and I certainly can't convince anyone else.
It's like laughter, perhaps: Try to dissect it and it disappears, although to say it disappears is clearly not true.