Sight can be demonstrated to the blind.
Hand to a blind man a bag of red marbles and a bag of blue marbles. Let him take them home and inspect them any way he wishes, so that he may determine them to be, to him, indistinguishable. The next day, have him construct for you in private and in any way he chooses a necklace out of the marbles. By guessing, against all odds, the exact sequence of of colors he chose, you may demonstrate to a blind man your sight.
