You have lost your left arm but you still feel your left hand. You not only feel it, 
but every minute of the day you are are nagged by an exhausting need to scratch the hand. But there is nothing there to scratch.How can this be? You feel something in the middle of nothing but air. You look down and you see empty space, but you still feel an extremely annoying itch right there where your hand is supposed to be.
You have a problem and it looks like there is no way to solve it. You can't scratch something that does not exist.
