If all his moves were revocable....

"In a game of chess you can make certain arbitrary concessions to your opponent, which stand to the ordinary rules of the game as miracles stand to the laws of nature. You can deprive yourself of a castle, or allow the other man sometimes to take back a move made inadvertently. But if you conceded everything that at any moment happened to suit him - if all his moves were revocable and if all your pieces disappeared whenever their position on the board was not to his liking - then you could not have a game at all." -- C S Lewis, The Problem of Pain 1940