The Republic (380 BC) by Plato is a Socratic dialogue, concerning the definition of justice, the order and character of the just city-state and the just man. As Plato's best-known work, it has proven to be one of the world's most influential works of philosophy and political theory, both intellectually and historically. In it, Socrates along with various Athenians and foreigners discuss the meaning of justice and examine whether or not the just man is happier than the unjust man by considering a series of different cities. The participants also discuss the theory of forms, the immortality of the soul, and the roles of the philosopher and of poetry in society.