Before he can prove that justice is a good thing, Plato must first state what justice is. Instead of defining justice as a set of behavioral norms (as the traditional Greek thinkers did) Plato identifies justice as structural: political justice resides in the structure of the city; individual justice resides in the structure of the soul. The just structure of the city is summed up by the principle of specialization: each member of society must play the role for which his nature best suits him and not meddle in any other business. A man whose nature suits him to farming must farm and do nothing else; a man whose nature best suits him to building objects out of wood must be a carpenter and not bother with any other sort of work. Plato believes that this is the only way to ensure that each job is done as well as possible.

The principle of specialization keeps the farmer from carpentering, and the carpenter from farming. More important, it keeps both the farmer and the carpenter from becoming warriors and rulers. The principle of specialization separates society into three classes: the class of producers (including farmers, craftsmen, doctors, etc.), the class of warriors, and the class of rulers. Specialization ensures that these classes remain in a fixed relations of power and influence. Rulers control the city, establishing its laws and objectives. Warriors carry out the commands of rulers. Producers stay out of political affairs, only worrying themselves about the business of ruling insofar as they need to obey what the rulers say and the warriors enforce. A city set up in this way, Plato contends, is a just city.