The conventional view of justice makes is a mean, mediocre thing: it is the mean between doing what is best – doing injustice without penalty – and what is worst – suffering injustice without being able to avenge oneself. In short, it is best to take stuff and worst to have stuff taken. This makes the practice of justice the result of the inability to practice injustice. It is the social compact of the sheep against the wolves and lions.
To illustrate this position more forcefully, Glaucon develops the Ring of Gyges myth, wherein he argues that were the just and the unjust man each to have a ring making them selectively invisible, you would find the just man doing the same thing as the unjust man out of the desire to get the better: