Plato's Republic is considered by many to be his greatest dialogue, and its influence on Western thought has been vast. In this rich, ambitious work, Plato sets out theories of reality, knowledge, human nature, politics, ethics, education, and the arts. The Republic also contains several unforgettable mythic stories. Among the most important of these is the Myth of the Cave, which allegorizes Plato's views about reality, knowledge and philosophy, and the Myth of Er, which purports to be the story of someone who has been shown what the afterlife is like and has come back to tell about it.

The two leading questions of the Republic are "What is justice?" and "Is the just life only valuable as a way of avoiding social unrest or being shunned or punished, or is it intrinsically valuable even without external rewards?" The first question is introduced and debated early in Book One, where conventional answers are examined and found wanting. Thrasymachus, one of the most interesting opponents Socrates debates in Plato's dialogues, introduces the second question by first trying to redefine justice in terms of power and by then claiming that no one would be just if they were strong enough to get away with being unjust. Thrasymachus is outmaneuvered by Socrates and acquiesces, at least verbally, to Socrates' argument in favor of justice. However, Book One ends with Socrates saying that he has been too hasty, rushing to praise justice before ever clearly answering the question of what justice is.

In Book Two, Socrates is challenged to undertake the task of clarifying the nature of justice and vindicating its intrinsic value. Glaucon and Adeimantus (two characters in the dialogue who happen to be Plato's brothers in real life), undertake being devil's advocates for Thrasymachus's jaded views. The brothers think that many people (who would not say so out loud) really do regard justice as an inconvenient matter that they attend to unwillingly out of fear of being caught. Glaucon tells the story of the shepherd Gyges who, after finding a ring which confers invisibility, uses this power to gain absolute political power unjustly. The story raises the question of whether all of us would be unjust when it is to our advantage if we had a "cloak of invisibility."

Socrates takes up what we might call "Glaucon's Challenge." He aims to show what justice is and that being just is valuable apart from producing social harmony and a good reputation. The rest of the Republic is concerned with Socrates' attempt to fulfill this challenge.

The first thing, then, that Socrates must do is make clear what justice really is. He argues that a polis (a city-state, or what we might call a society, nation-state, or government) has a structure and that this structure is parallel to the structure of an individual soul. In other words, justice is a virtue both of societies and of individuals. If justice can be identified and understood in the state then we can identify and understand it in the individual. These claims allow Plato to set forth both his political philosophy and his ethical philosophy at the same time.

In the process of setting out his political and ethical theory, Plato has Socrates make a number of startling claims. Interpreters of the Republic disagree about which of these claims represent Plato's considered views and which are made for rhetorical effect. Among the claims that Socrates makes are the following. If people with the right sort of breeding are given the right sort of education and the right sort of testing, they can be trusted with absolute political power when they are mature adults. Good breeding is so important to producing good leaders that mating should be regulated by the state (at least among the ruling class). Private families and private property should be forbidden among the ruling class. Women who display potential for leadership should be given the same education and training as men and should be given the same political power. While deception should be absolutely forbidden among ordinary citizens, those who govern should lie to the citizens occasionally for their own good. What most people consider knowledge is really closer to ignorance than to reality. Philosophers are those best suited to acquire true knowledge and are thus those best suited to govern. Being just is so intrinsically valuable that a just person is better off than an unjust person, even if the just person is misunderstood, punished, or even killed.

The society that Plato describes in the Republic is often thought of as the first utopia. A utopia, which means literally "no where," is a depiction of a society so ideal that it exists no where on earth. The reader is reminded several times that the real goal of this dialogue is to vindicate the intrinsic value of being a just person. Socrates at points indicates that any political claims being made are secondary to that task and that it doesn't much matter whether a society like the one described is a practical possibility. Plato's attitude toward the society thus described remains ambiguous. Modern westerners have often borrowed features of his republic as elements for anti-utopian novels (e.g. 1984, Brave New World). This is because Plato's "ideal" society sacrifices individual freedom to the common good and is deeply anti-democratic.

Toward the end of the Republic, Socrates says that "each of us must neglect all other subjects and be most concerned to seek out and learn those that will enable him to distinguish the good life from the bad and always make the best choice possible in every situation." Whatever view one takes of interpretive questions surrounding the text, and whether one is attracted or repelled by the picture of life Plato presents, it is clear that the Republic offers banquet-sized portions of food for thought concerning what the good life involves.