54. Let us recall the kinds of case where we say that a game is played according to a definite rule. The rule may be an aid in teaching the game. The learner is told it and given practice in applying it.—Or it is an instrument of the game itself.—Or a rule is employed neither in the teaching nor in the game itself; nor is it set down in a list of rules. One learns the game by watching how others play. But we say that it is played according to such-and-such rules because an observer can read these rules off from the practice of the game—like a natural law governing the play.—But how does the observer distinguish in this case between players' mistakes and correct play?—There are characteristic signs of it in the players' behaviour. Think of the behaviour characteristic of correcting a slip of the tongue. It would be possible to recognize that someone was doing so even without knowing his language.