The Republic is a Socratic dialogue, written by Plato around 380 BC, concerning justice, the order and character of the just city-state and the just man. It is Plato's best-known work, and has proven to be one of the world's most influential works of philosophy and political theory, both intellectually and historically. In the book's dialogue, Socrates discusses the meaning of justice and whether or not the just man is happier than the unjust man with various Athenians and foreigners. They consider the natures of existing regimes and then propose a series of different, hypothetical cities in comparison. This culminates in the discussion of Kallipolis, a hypothetical city-state ruled by a philosopher king. They also discuss the theory of forms, the immortality of the soul, and the role of the philosopher and that of poetry in society.