The Ring of Gyges
If you had a ring that made you invisible and completely undetectable, with no consequences ever, would you still behave morally?
Plato introduced this scenario in the Republic around 380 BC, through the character of Glaucon, to challenge Socrates. Glaucon argues that everyone would abuse such power, and that therefore morality is just a social contract we follow out of fear, not genuine virtue.
Plato. (c. 380 BC). Republic, Book II. Trans. G.M.A. Grube.
The original argument
Glaucon tells the story of Gyges, a shepherd who found a ring with the power of invisibility in a chasm opened by an earthquake. Gyges used it to seduce the queen, kill the king, and take the throne.
Glaucon's point: any rational person would do the same. Justice is not chosen for its own sake. It's chosen because of social rewards and the fear of punishment. Remove the consequences, and the "just" person and the "unjust" person make the same choices.
Socrates' challenge: prove Glaucon wrong.
Socrates' reply
Plato has Socrates argue that injustice corrupts the soul. A person who uses the ring for unjust ends doesn't gain. They damage their own inner harmony, becoming less capable of reason, wisdom, and genuine happiness. The just person is happy; the unjust person is disordered, even when rich and powerful.
This is Plato's theory of justice as a property of the soul, not just behavior. You should be just not to avoid punishment but because justice is constitutive of human flourishing.
Does that satisfy you?
Most people find Plato's reply unconvincing as a direct argument. The unjust person can seem quite happy, thank you. But the scenario is still useful because it exposes what your theory of morality actually is. If you'd behave the same with the ring, you believe in intrinsic moral reasons: justice is worth pursuing regardless of consequences. If you'd use it, you believe moral behavior is fundamentally social: it's about what happens when others are watching.
The real test
Not: would you do something dramatically evil. But: would you cut in line? Steal small things from large corporations? Bend the rules when only you'd know? The small uses of the ring reveal the actual structure of your moral commitments.