In The Republic, Plato hypothesizes the formation of the ideal city, the kallipolis, as a way to embody the philosophical definition of perfect justice. In the just city, one is most likely to see the execution of perfect justice, and so the composition of this city will be the best indicator of justice as an idea. For Plato, justice had a simple definition (with a much more complicated rationalization): justice is each person in the city doing only the work to which they are best suited:
“Then, it turns out that this doing one’s own work – provided that it comes to be in a certain way – is justice.” (433b)
The argument that justice is each doing one’s own work transitions naturally into Plato’s (speaking through Socrates) next one, that the kallipolis should be ruled by philosopher-kings. If each person in the city-state is only best suited to one kind of work, if they want to carry out that work perfectly, then only a very select few people will actually possess the aptitude and skills necessary for leading the city as its kings. Plato argues that these people are the philosophers.
The ideal society of course contains many other kinds of people. The guardians of the city must possess a perfect balance of temperance and courage, and they must be loyal only to their own city-state. In fact, they should behave as dogs do—for Plato argues that dogs love their owners unconditionally, even if they have only ever received abuse from them. This is because dogs know their owners, and do not know strangers, of whom they are always suspicious. Dogs, therefore, only form loyalties based on things that they know—on their knowledge—and are thus good models for the guardians of the city. The ideal city should also contain carpenters, poets, farmers, tradesmen, musicians, and every other kind of person who holds an occupation that gives the city its material and cultural life. However, these people must only engage in the craft to which they have the highest affinity.
But philosophers, above all other people, are fit to rule the kallipolis. The reason for this is because, according to Plato, the philosophers are the only people wise enough to act in accordance with the idea of justice. In Plato’s philosophy, the idea of a metaphysical concept, such as justice, love, beauty, etc., was that concept abstracted from any of its physical manifestations in the sensible world. One does not speak of the true, unchangeable concept of beauty, for example, if they are talking about the individual beauty of a person or of nature. This is because, over time, the things in the physical world that represent beauty (or any other concept) change, as do individuals’ perceptions of them, and are therefore impermanent.
Plato argued that true beauty, or justice, are never-changing, eternal, and allow things in the physical world to have such attributes. This inveterate form of the metaphysical concept of Justice is what Plato referred to as the “Idea” of it, and he further argued that only the philosophers were capable of understanding the Ideas. Because of this unique ability of theirs, the philosophers were the only individuals capable of making decisions and acting on a form of Justice that was eternal and pure, and they would therefore not be capricious, arbitrary, or cruel in their administration of the city.
This social composition, and the philosophic rationalization that lie behind it, were the most important features of Plato’s ideal city.