The Republic

In the Republic, Socrates discusses the meaning of justice and whether the just man is happier than the unjust man with various Athenians and foreigners. He considers the natures of existing regimes and then proposes a series of hypothetical cities in comparison, culminating in Kallipolis, a utopian city-state ruled by a class of philosopher-kings. They also discuss aging, love, the theory of forms, the immortality of the soul, and the role of the philosopher and of poetry in society.
The Republic is generally placed in the middle period of Plato's dialogues. However, the distinction between this group and the early dialogues is not as clear as the distinction between the late dialogues and all the others. Nonetheless, with their separate methodologies, Ritter, Arnim, and Baron agreed that the Republic was well distinguished, along with Parmenides, Phaedrus, and Theaetetus.
However, the first book of the Republic, which shares many features with earlier dialogues, is thought to have originally been written as a separate work. Then the remaining books were conjoined to it, perhaps with modifications to the original of the first book.