In Plato's republic the healthy city is explained in the same terms as that of the healthy individual.explain this congruity?

In Plato's "Republic," the healthy city is a metaphor for the well-ordered individual. Just as the city requires each class (rulers, auxiliaries, producers) to perform its function harmoniously for the city to thrive, the individual's soul must have reason ruling over spirited and appetitive parts for a balanced and virtuous life. This congruity between the city and individual reflects Plato's belief that the principles governing a just city are the same as those governing a just individual.