Thought experiment introduced by the character Glaucon, in Plato’s Republic, Book II, 359d. The ring rendered its wearer Gyges, a shepherd, invisible, enabling him to commit crimes with impunity. Gyges used the cloak of invisibility to seduce the King’s wife and then with her help usurp the kingdom. Glaucon uses the story to make vivid the thought that only fear of penalties deters people from behaving unjustly.