In book 3 of Plato's Republic, Socrates continues to describe his ideal city by outlining how its guardians would be educated. They should be told stories of simple heroism, without evil characters or dishonest conduct, since these will provide them with models to imitate. Their teachers should love the children for their goodness and should help them to lead pure and simple lives. The future guardians should be trained as soldiers, and any who are physically or mentally unfit for their duties should be allowed to die.

Only the best of these children will become guardians of the city. The remainder will be their assistants and soldiers. The guardians must be totally devoted to the city. Socrates divides humanity into three categories: those fitted to rule have souls of gold; those who should be warriors have souls of silver; and those who do manual work have souls of bronze or iron. The city will be destroyed if it is not ruled by someone with a soul of gold. The guardians of the city will have no personal possessions and will not be allowed to touch gold or silver, since this would pollute their golden souls.