Glaucon’s challenge is to show why justice is valued “for its own sake.” (The quotation marks are there because it isn’t obvious exactly what it means to value something “for it’s own sake.”) That, in turn, requires answering a second question: what is justice?
Plato believed that he could give one answer to this question that would apply to both cities and individuals. That is, he thought that we could learn what it is for an individual to be just by looking at what makes a city just and vice versa.
Guardians were introduced to solve some of the luxurious city’s needs. (Plato later split this group between guardians, who rule, and auxiliaries, who make up the military force supporting the guardians.) Having introduced a military class, Plato set about describing the selection and education of its members. It’s important to select good leaders and also to train them so that they will stick to their jobs rather than exploiting their power.
One interesting thing about this is that Plato hasn’t yet described why even the luxurious city needs leaders, people who govern the behavior of its members. The need for guardians came entirely from relations with people outside the city. We’ll get more of the story later; my only point is that we haven’t heard much yet.
Another interesting thing is that the myth of the metals is directed more at the guardians and auxiliaries than the rest of the city. Now, Plato did say that the myth would be told to “the rulers and the soldiers and then the rest of the city (414d). So it’s going to be addressed to everyone. But Plato only described the effects of the myth on the guardians. We’re familiar with false beliefs that have the function of discouraging members of a society from pursuing their own interests. But these are usually directed at those closer to the bottom of society with the aim of getting them to accept their lower status.
Furthermore, as Callum pointed out, it’s hard to understand how this myth is supposed to work on the guardians. The guardians are supposed to know the truth. They’re put in charge because they are much better than everyone else at doing this one thing. But if they’re so smart, inquisitive, and knowledgeable, why are they going to fall for this rather hokey tale?
Plato will try to show that life in the just city is the best life for all concerned. So this sort of lie isn’t exploitative in the way that the familiar falsehoods that we’re typically concerned with are. It’s an interesting question whether “noble,” non-exploitative, lies like this play a legitimate role in our political lives or not. It’s a question that we’ll return to when we talk about Sidgwick’s utilitarianism and Rawls’s liberalism.
Here are some terms that should be familiar to you.