Plato advances many of his principal ideas concerning government and justice in his Republic, written about 370.

Set forth as a debate between Socrates and five other speakers, the work describes an ideal state (based on Sparta), which Plato argues should be ruled by philosopher-kings.

Plato regards the philosopher as best suited to govern because he perceives the natural harmony between the principles of justice that govern the state and those that rule the individual.

Plato presents his concept of the ideal Forms in Book Seven’s allegory of the Cave—the world of illusion and ignorance—beyond which only the philosopher has ventured to perceive the ideal models of justice.

His doctrine of Forms postulates a realm of pure ideas, or essences, that exists above and beyond the illusory sensory world.

Plato's nephew and disciple, Speusippus, around 370 devises an encyclopedia-like work to record permanently the master's lectures on mathematics, natural history, and philosophy.