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How does Plato define justice in the Republic?

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The Greek philosopher Plato — Plato Plato, marble portrait bust copied from an original of the 4th century bce; in the Capitoline Museums, Rome.

In his work The Republic, Plato defines justice as a state of proper order and harmony, which he examines through both political and ethical lenses.12

Political Justice

Plato defines political justice as the condition of an ideal city-state where each of the three social classes—rulers, guardians, and producers—performs its specific role without interfering with the functions of the others. The rulers, who are philosophers, govern based on their knowledge of the form of the Good and the form of the Just.2

Ethical Justice

Ethical justice in the individual is analogous to this political structure. It is defined as a state of psychic harmony within the soul, which consists of three parts: reason, spirit, and appetite. Justice occurs when each part performs its proper role:2
* Reason seeks truth and the good of the whole individual.2
* Spirit and appetite are guided by reason, desiring only what is healthy and proper.2

Ultimately, Plato argues that justice is in each person’s best interest, as it represents the ideal functioning of both the state and the individual.12