Plato is the grandfather of philosophy and The Republic is his magnum opus. It is one of the Socratic dialogues, in which Socrates discusses the nature of the City-State with fellow Athenians. Other topics of the dialogue include ideas of justice, goodness, reality, education, and knowledge. The essential concept in this work is that societies mirror the men and women who create them, so an ideal society would be modeled after a corresponding ideal human. Thus, in Plato's mind, society should be ruled by "philosopher kings." The Republic is basically the Big Bang of philosophy, so If you haven't read much of it, this work is a great place to start.