Plato was born in Athens in 428 B.C. to an aristocratic family. After the political upheavals in Athens during his youth and the death of his mentor, Socrates, Plato turned away from politics and founded the Academy in Athens in 387 B.C., which taught students from all over the Mediterranean in various fields. Plato spent the rest of his life at the Academy, where he wrote The Republic in around 380 B.C. and trained famous students such as Aristotle. Plato's political theory in The Republic was intended to be put into practice, but he only left the Academy twice, both times to visit Sicily. In 367 B.C., Dionysus I, the tyrant of Sicily, died, and Dion, father of the heir and Plato's student, sent for Plato. However, Dionysus II did not believe that the study of mathematics and philosophy would be the best preparation for his rule, and so the opportunity was lost to test Plato's political theory.

In "The Republic," Plato seeks to define justice and show that it is valuable for its own sake. Plato first defines political justice as the harmony of a structured political body consisting of three classes of people: producers, auxiliaries, and guardians. Each group must perform its appropriate function and be in the right position of power in relation to the others.

Justice is a principle of specialization that requires each person to fulfill the societal role to which nature fitted him and not interfere in any other business. Plato then shows that individual justice mirrors political justice, and the soul of every individual has a three-part structure analogous to the three classes of a society. In a just individual, the rational part of the soul rules, the spirited part of the soul supports this rule, and the appetitive part of the soul submits and follows wherever reason leads.

Plato uses a series of three analogies, including the allegories of the sun, the line, and the cave, to explain his theory of Forms and the philosopher's role in society. The philosophers are the only class of people who possess knowledge and are the most just. Plato argues that justice is worthwhile for its own sake, and a just soul is a healthy and happy one, untroubled by internal conflict, whereas injustice tortures a man's psyche.