The "Ring of Gyges" story is a sort of thought experiment. It occurs in the context of a discussion about justice in which Thrasymachus has just argued that, essentially, justice is whatever is in the interest of the strong. Socrates has forced Thrasymachus to reluctantly retreat from this position, and Glaucon takes up the argument with his story of the ring. In short, the ring of Gyges has the power to make its wearer invisible. When Gyges, an honest shepherd, finds the ring, he almost immediately begins to act in ways he would not have done previously. In particular, he conspires to seduce the queen and overthrow the king, seizing the kingdom for himself.
In telling this story, Glaucon is arguing that men behave in just ways not because they are inherently just, but because they will face consequences for behaving unjustly:
No man would keep his hands off what was not his own when he could safely take what he liked out of the market, or go into houses and lie with any one at his...
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