"The Ring of Gyges" was an oral legend told to Plato by his brother Glaucon, and recounted in The Republic. The story concerned a magic ring that made its wearer invisible; this allowed the shepherd Gyges to seduce the Queen of Lydia and assume power after killing the King. The story is an example of a morality tale; the question is whether the power of anonymity would eliminate morality and ethics in any person, even one of very strong moral beliefs. Glaucon argued that two men with opposing moral views would be affected in different ways; the fully honest man would not use the ring's power for his own gain and so would be miserable, while the fully dishonest man would use the ring and be happy and fulfilled.
"If you could imagine any one obtaining this power of becoming invisible, and never doing any wrong or touching what was another's, he would be thought by the lookers-on to be a most wretched idiot, although they would praise him to one another's faces..."
(Plato, The Republic)
The moral of the story is that power will cause any man, even an honest one, to become dishonest, or at least motivated by selfish desires. Glaucon's comment that a person who never used the ring selfishly would be mocked behind his back is interesting, as it seems that those mockers would be fearful of retribution if they spoke aloud even though the ring-bearer never used the ring badly. This speaks to the flawed nature of human moral philosophy; most ethical questions can be confused with semantic arguments and excuses.