The Old Woman and the Doctor explained simply

An Old Woman became almost totally blind from a disease of the eyes, and, after consulting a Doctor, made an agreement with him in the presence of witnesses that she should pay him a high fee if he cured her, while if he failed he was to receive nothing. The Doctor accordingly prescribed a course of treatment, and every time he paid her a visit he took away with him some article out of the house, until at last, when he visited her for the last time, and the cure was complete, there was nothing left. When the Old Woman saw that the house was empty she refused to pay him his fee; and, after repeated refusals on her part, he sued her before the magistrates for payment of her debt. On being brought into court she was ready with her defence. "The claimant," said she, "has stated the facts about our agreement correctly. I undertook to pay him a fee if he cured me, and he, on his part, promised to charge nothing if he failed. Now, he says I am cured; but I say that I am blinder than ever, and I can prove what I say. When my eyes were bad I could at any rate see well enough to be aware that my house contained a certain amount of furniture and other things; but now, when according to him I am cured, I am entirely unable to see anything there at all."