The soul has three parts: reason, which calculates and seeks truth; spirit, which feels anger, honor, and indignation; and appetite, which hungers for food, drink, sex, and money. Justice in a person is each part doing its own work with reason ruling, spirit enforcing, and appetite obeying; injustice is any part usurping a role not its own.
Justice is not primarily about external actions or reputation but about the right internal arrangement in which each part of a thing performs its proper function and does not meddle in others'. This holds in parallel for the city (rulers rule, guardians defend, producers produce) and for the individual soul, so justice is a kind of structural health rather than a social contract.
Prisoners chained in a cave take shadows cast on a wall for reality; one freed prisoner is dragged up into sunlight, is pained and disoriented, and only gradually comes to see real things and finally the sun itself. Returning to tell the others, he is mocked and resented, illustrating that education reorients the whole soul toward truth and that the enlightened are rarely welcomed back.
A city will have no rest from evils until political power and philosophy coincide in the same persons, because only those who have grasped the eternal forms know the good they are arranging society toward. Rule should go to those who least want it and are least corruptible by it, not to those who seek it.
To read justice in the individual, construct it first in the larger, more legible unit of a city, where the same structure appears at readable scale. What looks like a political arrangement of classes is actually a diagram of the internal faculties competing for rule within one person.
Constitutions degenerate in sequence as the dominant human motive changes: aristocracy (reason) decays into timocracy (honor), then oligarchy (wealth), then democracy (free appetite), then tyranny (a single lawless desire). Each regime is mirrored by a corresponding type of person, so political decline tracks a predictable disordering of the soul's priorities.