Socrates declares the just city complete. Since this city has been created to be the best city possible, we can be sure that it has all the virtues. In order to define these virtues, all we need to do is look into our city and identify them. So we will now look for each of the four virtues: wisdom, courage, moderation, and justice.- Ch IV Summary, Plato's "Republic"
We find wisdom first. Wisdom lies with the guardians because of their knowledge of how the city should be run. If the guardians were not ruling, if it were a democracy, say, their virtue would not translate into the virtue of the city. But since they are in charge, their wisdom becomes the city’s virtue. Courage lies with the auxiliaries. It is only their courage that counts as a virtue of the city because they are the ones who must fight for the city. A courageous farmer, or even ruler, would do the city no good. Moderation and justice, in contrast to wisdom and courage, are spread out over the whole city. Moderation is identified with the agreement over who should rule the city, and justice, finally, is its complement—the principle of specialization, the law that all do the job to which they are best suited.
So now we have reached one of our two aims, at least partially. We have identified justice on a city-wide level. Our next task is to see if there is an analogous virtue in the case of the individual.