Excerpt from Plato’s Republic:

Gyges was a shepherd in the service of the ruler of Lydia. One day there was a violent thunderstorm, and an earthquake broke open the ground and created a crater at the place where Gyges was tending his sheep. Seeing the big hole, Gyges was filled with amazement and went down into it. And there, in addition to many other wonders of which we are not told, he saw a hollow bronze horse. There were window-like openings in it, and peeping in, he saw a corpse, which seemed to be of more than human size, wearing nothing but a gold ring on its finger. He took the ring and came out of the crater.

Gyges wore the ring at the usual monthly meeting that reported to the king on the state of the flocks of sheep. As he was sitting among the others, he happened to turn the setting of the ring towards himself to the inside of his hand. When he did this, he became invisible to those sitting near him, and they went on talking as if he had gone. He wondered at this, and, fingering the ring, he turned the setting outwards again and became visible. So he experimented with the ring to test whether it indeed has this power – and it did. If he turned the setting inward, he became invisible; if he turned it outward, he became visible again. When he realized this, he at once arranged to become one of the messengers sent to report to the king. And when he arrived there, he quickly became the queen’s lover. With her help he attacked the king, killed him, and took over the kingdom.

Plato has us imagine two magic rings, one given to a morally good person and one to a person who is not morally good. Plato contends that there would be no difference in the way the two would behave. People only behave morally, he claims, because they are afraid of the consequences of getting caught if they behave badly.