Sometimes it takes years of debate for philosophers to discover what it is they really disagree about. Sometimes they talk past each other in long series of books and articles, never guessing at the root disagreement that divides them. But occasionally a day comes when something happens to coax the cat out of the bag. "Aha!" one philosopher exclaims to another, "so that's why you've been disagreeing with me, misunderstanding me, resisting my conclusions, puzzling me all these years!"