The Republic. Plato's Dialogue on Justice, the Ideal State, and What a Good Life Actually Requires

Socrates and a small group of Athenians set out to answer a simple-sounding question - what is justice? - and end up designing an entire ideal city-state from first principles, debating the nature of knowledge, the allegory of the cave, and whether philosophers or soldiers or merchants should actually be in charge. Written as conversation rather than lecture, it remains the most readable entry point into the questions that founded Western philosophy. The single most influential work of philosophy ever written - the direct starting point for political theory, ethics, and epistemology as fields of study.