The Cobbler Turned Doctor explained simply

A very unskilful Cobbler, finding himself unable to make a living at his trade, gave up mending boots and took to doctoring instead. He gave out that he had the secret of a universal antidote against all poisons, and acquired no small reputation, thanks to his talent for puffing himself. One day, however, he fell very ill; and the King of the country bethought him that he would test the value of his remedy. Calling, therefore, for a cup, he poured out a dose of the antidote, and, under pretence of mixing poison with it, added a little water, and commanded him to drink it. Terrified by the fear of being poisoned, the Cobbler confessed that he knew nothing about medicine, and that his antidote was worthless. Then the King summoned his subjects and addressed them as follows: "What folly could be greater than yours? Here is this Cobbler to whom no one will send his boots to be mended, and yet you have not hesitated to entrust him with your lives!"