There is a story told by Glaucon in the second book of Plato's Republic, where Glaucon and his brother Adeimantus are trying to get Socrates to explain to them why being just to others is an aid to leading a happy life. They tell him the story of Gyges--Gyges was a servant in the household of the King of Lydia, and Gyges fell in love with the Queen and the Queen with him. The Queen gave Gyges a ring which if he turned it around, made him invisible. Gyges used the ring, made himself invisible, murdered the King and married the Queen, and became the ruler of Lydia.
Suppose there's only one ring of Gyges in the world and you're walking up Fifth Avenue on a bright sunny morning, and you pass Tiffany's at 5th Avenue and 57th Street and in one of those little windows, you see a little black case with a nice gold ring and a sign under it says, “Ring of Gyges, moderately priced, inquire within.”
One, would you go in and inquire about the price of the ring? Two, if it was within your power to buy it, would you buy it? Three, if you bought it, how would you use it? Remembering that the ring of Gyges would enable you to do anything you wanted and have no fear of ever being caught or punished.