The question of the Republic
Justice is the question around which Plato's Republic is built. The dialogue opens with the challenge of the sophist Thrasymachus — that justice is nothing but "the interest of the stronger," a convention imposed by the powerful for their own advantage — and the whole vast construction that follows is Plato's answer. The platform reads justice in Plato as one of the most ambitious accounts of the concept in the history of thought: not a rule of conduct or a social convention but a structural condition of the soul and the city alike.
Justice as right ordering
The platform reads Plato's definition as the analogy between soul and city. The soul has three parts — reason, spirit, and appetite — and the city three classes — rulers, guardians, and producers; justice in each is the same thing, each part doing its own work under the rule of reason, and not meddling in the work of the others. The just soul is the one in which reason governs, spirit supports it, and appetite obeys; the just city is the one in which the wise rule, the brave defend, and the productive provide, each in its place. The platform reads this under the ideal state: justice is harmony, the right relation of parts to whole.
Why justice pays
The platform reads the deepest aim of the Republic as proving that justice is worth choosing for its own sake — that the just person is happier than the unjust, even one who escapes all punishment and enjoys every reward of injustice. Against the sophistic and popular view that justice is a burden the clever evade when they can, Plato argues that injustice is the disorder and disease of the soul, and that no external good can compensate for it. The platform reads the Gorgias as the sharpest form of this argument: it is better to suffer injustice than to do it, because the wrongdoer harms the most important thing he has, his own soul.
Why it matters for Virtue & Power
Justice in Plato is the foundation of his political thought and one of the platform's central treatments of the concept, to be read against the realist account in Thucydides and the practical account in Aristotle. It is the deepest answer the classical tradition offers to the question the sophists posed — whether justice is anything more than the interest of the strong — and it underwrites the whole of Plato and political order.