We hear of the Ring of Gyges in book II of the Republic, in which Socrates is discussing justice with a group of young men in the house of Cephalus at the Piraeus. Socrates has just easily refuted definitions of justice given by Cephalus, Polemarchus, and Thrasymachus and is eager to leave. Glaucon however is unsatisfied with these refutations, for Socrates was merely talking at the level of these simpletons. Glaucon demands Socrates to stay and discuss justice further. He begs of Socrates, why should justice be good and desirable as an end of itself if men only offer praise and reward those who merely seem just? For illustration, Glaucon thus recounts the story of Gyges:*

Gyges was a shepherd in the service of the king of Lydia. One day as Gyges was attending his to his flock there occurred a mighty storm. In the height of its intensity there was an earthquake which shook the earth violently, opening a chasm in the place where Gyges was feeding his flock. In the rift Gyges beheld, among other marvels a hollow brazen horse. Inside of this horse he saw a dead body of stature, wearing nothing but a gold ring. Gyges took from him this ring and returned to his flock.

The shepherds congregated so that they might send their monthly report regarding the flocks to the king. As Gyges sat amongst them he merely fidgeting, turned the collet of the ring inward on his finger. Instantaneously, he became invisible to the rest of the company and they began to speak of him as if he were no longer present. Gyges was astonished and turned he ring to its original position to find himself reappear. With this ring in his possession, Gyges schemed to be chosen as messenger to the court of the king. When he arrived he seduced the queen and with her aid conspired against the king and slew him… the kingdom of Lydia now his.

Why should a man, if he were to possess such a ring, desire to fare justly? He would suffer no punishment from others for his unjust actions for they would be committed while he remained invisible. Why is the just life the best? Thus Glaucon asks, and the discussion continues for the rest of Plato’s Republic.

* Paraphased from Benjamin Jowett's 1901 translation of The Republic, which is now in the public domain.