“[T]hough in all practical matters it is indispensable, either always or mostly, to follow custom, to do what is generally done, in theoretical matters it is simply untrue. In practical matters there is a right of the first occupant: what is established must be respected. In theoretical matters this cannot be. Differently stated: The rule of practice is 'let sleeping dogs lie,' do not disturb the established. In theoretical matters the rule is 'do not let sleeping dogs lie.' Therefore, we cannot defer to precedent . . . .”