The Republic, by Plato. Part II.

The Republic is a Socratic dialogue written by Plato around 380 BC concerning the definition of justice and 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 most intellectually and historically influential works of philosophy and political theory. 

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 coming into existence "in speech", culminating in a city (Kallipolis) ruled by philosopher-kings; and by examining the nature of existing regimes. The participants also discuss the theory of forms, the immortality of the soul, and the roles of the philosopher and of poetry in society. (From Wikipedia.)

Translation by Benjamin Jowett.