Plato says that there are three parts of the soul: the desiring, the spirited, and the calculating. He says that “the one within whom each of the parts minds its own business will be just and mind his own business” (441e). Each part of the soul minds its own business by ruling when it is most likely to do justice, and being ruled obediently when it is most likely to do injustice. Since the desiring part of the soul is “surely most of the soul in each” (442a), and since when it is fulfilled it means the end of moderation, it is the most likely of the parts to do injustice. It must thus be ruled over by both the calculating and spirited parts of the soul. Since the calculating part is “wise and has forethought about all of the soul” (441e), its business is to also rule the spirited part, although the two should work together. The man in which each part of the soul does its business, ruling or being ruled correctly, will master himself and achieve justice.
Plato suggests that the justice in ones soul does not only reflect the justice in a city, it is essential to it. When one is just inside, he contributes to a just city. The calculating part of the soul reflects the philosophical people in a city, but that calculating soul itself is also crucial in the city’s justice; Plato dubs those with it rulers and guardians. Each part of the soul shows itself in the city; the people with dominantly spirited souls are warriors, the people with dominantly desiring souls are the normal people who need to be lead. Plato argues that a just city has the correct balance of all three parts, and so does the man who has mastered himself and achieved justice. In order for the just city to have the three types of people, however, it needs men who have the three parts to their souls. A city cannot achieve justice, then, without people who have organized these three parts correctly: people who have mastered themselves.

-Kathryn D