Dikaiosynē

Dikaiosynē (δικαιοσύνη) is justice, or righteousness — the virtue of rendering to each its due. It grows from dikē, an old and weighty word meaning right, judgment, and the settled order of things, which the Greeks also personified as a goddess. Of the four cardinal virtues, justice is for Plato the one that binds the others into a whole.

The question of the Republic

'What is justice?' is the question that launches the Republic, and Plato's answer is famously unexpected. Justice is not, at bottom, a matter of rules or reputation but of right structure. In the city it is each class doing its own proper work and not meddling in another's; in the soul it is the parallel order in which reason rules, the spirited part defends, and the appetites obey.

The health of the soul

On this view justice is the health, harmony, and beauty of the soul, and injustice its disease and disorder. This is why Plato can argue, against those who praise the profitable life of the unjust, that the just person is happier even on the rack than the tyrant on his throne: justice is the soul's own well-being, choiceworthy for its own sake and not merely for its rewards. It is inseparable from excellence, temperance, and the rule of the Good.