On his return from a religious festival, Socrates encounters Polemarchus and returns with him to the house of his father, Cephalus, where the three men discuss justice. Both Cephalus and Polemarchus give traditional accounts of what justice is, which Socrates shows to be incomplete. Thrasymachus enters the debate, answering that the very conception of justice is a sham meant to keep the strong at bay. True justice, he contends, is the advantage of the stronger. Socrates tries to rebut Thrasymachus’s claim, but Thrasymachus remains unconvinced. Book I ends at this point, and the remaining nine books consist of Socrates working out, in dialogue with Glaucon and Adeimantus, a more robust definition of justice.

Glaucon and Adeimantus urge Socrates to prove that justice is good in itself and not only for its consequences. People act justly mostly out of fear of punishment, so if justice is not good in itself, and if they thought they could get away with it, people would have no reason not to act unjustly. Rather than answer their question directly, Socrates proposes to sketch out an ideal republic so they can determine what role justice plays in this republic.

Socrates proposes a principle of specialization, according to which each citizen has a particular role to play. A city needs producers, who produce food and shelter, as well as a class of guardians who protect the state’s interests. These guardians are raised according to a rigorous program of education that emphasizes physical fitness, honor, and wisdom. They are shielded from bad influences, such as myths that portray the gods as possessing vices, so that they don’t become brutish or soft. The best among the guardians are selected as rulers (also referred to as “guardians”), while the others become “auxiliaries,” who act as soldiers. To maintain this strict class structure of producers, auxiliaries, and guardians, Socrates invents a state-sanctioned mythology that discourages people from aspiring to a different class. Class mobility is only possible when a youth in one class is identified with abilities that clearly suit him for a different class.

Socrates identifies the four primary virtues in the different aspects of this republic: the guardians possess wisdom, the auxiliaries possess courage, and the whole possesses justice and moderation. Thus, the justice of an ideal republic does not reside in any particular part of the republic but rather in the structure of the republic as a whole.

Like the just city, the soul of a just person is divided into three parts, and the soul’s justice resides in the proper structuring of these parts. The soul has an appetitive part that desires money and other earthly goods, such as the producers; a spirited part that desires honor, such as the auxiliaries; and a rational part that desires truth, such as the guardians. The rational part rules in a just soul, ensuring the health of the whole.

The guardian class lives austerely, having no money or material possessions. They live communally, choose sexual partners by lot, and are separated from their children at birth so as to prevent family ties from overriding loyalty to the state. In a move that is revolutionary for its time, Socrates sees no reason why women should not have status equal to men.

The guardians are philosopher-kings, not to be confused with contemporary philosophers, who are more accurately called “lovers of sights and sounds.” These lovers of sights and sounds are drawn only to the appearance of things, whereas true philosophers have knowledge of the unchanging, eternal Forms that lie behind appearance. The world of sights and sounds consists of objects that both are and are not—for example, a beautiful woman is both beautiful and, in comparison to a goddess, not beautiful. Therefore, the things we see and hear are objects of opinion or belief; maybe they’re beautiful, maybe not. The world of Forms, such as the Form of Beauty, however, has being in an absolute sense, and these Forms are the objects of knowledge.

The highest knowledge to which the philosopher-kings aspire is knowledge of the Form of the Good. Socrates cannot articulate directly what this is but rather explains it by offering three analogies: the sun, the line, and the cave. Socrates invites us to imagine prisoners chained to a bench in a cave. All they can see are the shadows moving on the wall in front of them, which are cast by statues being moved above and behind, where the prisoners cannot see. Not knowing any better, these prisoners think of the shadows as real, like a person seduced by the imaginative world of stories, unable to recognize a higher reality. If the prisoners were released, they could turn around and see that the shadows they thought were real were only projections of the statues behind them. They would then think of these statues as real, like a person who thinks the world of sights and sounds is the most real thing there is. The prisoners might then wander out of the cave and into the outside world. At first, they would be blinded by the light, but they would eventually come to see all the objects of the world around them. They would think of these objects as real, like a person who can grasp by means of thought the Forms that underlie everyday existence. Finally, these prisoners might be able to look at the sun itself and recognize it as the source of all light and all life. The sun is like the Form of the Good: just as the sun is the source of everything in the visible world, the Form of the Good is the source of everything in the intelligible world.

Socrates invites his friends to imagine a line divided first in two and then in four. The lower part represents the visible realm and the upper part represents the intelligible realm. The visible realm is divided into imagination and belief, belief being better than imagination just as seeing the statues is better than seeing the shadows. The intelligible realm is divided into thought and understanding, where thought hypothesizes the existence of Forms based on the visible world and understanding grasps the Form of the Good as a first principle from which everything else follows. The divided line is diagrammed in the following figure. The corresponding stages in the prisoner’s escape from the cave are in parentheses.

The education of the philosopher-kings is like the progress of a prisoner from out of the cave. In youth, they study mathematics to give them an intimation of an abstract world behind the visible. After rigorous physical training, they study philosophy and then dialectics. At thirty-five, they spend the next fifteen years running affairs of state before finally achieving the rank of philosopher-king at fifty. These philosopher-kings are like the prisoners who can see the sun, and contemplation of the Form of the Good will be their highest aim. However, they must also take care of the republic and train the next generation, just as freed prisoners must return to the cave to help their comrades.

Because the guardians will inevitably make some errors in judgment, this ideal republic will gradually decline through four stages of progressively worse government: timocracy, oligarchy, democracy, and tyranny. Similarly, a just man can slide into four kinds of degeneracy, tyranny being the worst. People who take whatever they can to please themselves live like tyrants, so people who follow Thrasymachus’s “advantage of the stronger” are worst off of all people. Only philosophers live just lives because only they have the ability to recognize the true pleasure to be found in the love of truth. All other pleasures are really just the cessation of pain.

Poets are banished from Socrates’ republic because they portray falsehoods and appeal to our emotions and baser instincts in a way that corrupts us. Socrates regrets this necessity and invites others to persuade him not to banish poets.

The last book of the Republic contains an argument for the soul’s immortality, claiming that injustice if anything would destroy a soul, and yet the soul seems to survive the tyranny of unjust men. Plato concludes with the myth of Er, a slain soldier who discovers that after death, good people spend one thousand years in heaven while bad people spend one thousand years in hell before selecting a new life for themselves.

The Republic is not so much a practical guide to future policy as it is a set of bold provocations. It is possibly the single most important philosophical work in the Western tradition, and the number of unconventional and bizarre views it contains is surprising. The ideas that men and women should be treated as equals and that justice is to be found within the structure of a state rather than in its actions were revolutionary in Plato’s day. Even two and a half millennia after its composition, no state has attempted the fifty-year educational process recommended for the guardians or the communal living that does away with the family and private property. By presenting these radical ideas within the framework of the ideal state, Plato challenges us to find reasons for faulting them. If we want to contradict these unconventional proposals, we will have to think as creatively as Plato has in formulating them.

The Republic contains less dialogue than Plato’s early work because it deals with such counterintuitive ideas. In dialogues such as the Euthyphro, we see Socrates discussing virtue and dismantling the various commonsense definitions of holiness, friendship, courage, and the like. The first book of the Republic works along similar lines, with Socrates dismantling the commonsense conceptions of justice held by Cephalus and Polemarchus. Things take a turn, however, when Thrasymachus dismisses justice as a whole, claiming that our very idea of justice has been imposed upon us by rulers who want to keep us in our place. The rest of the Republic can be read as a response to Thrasymachus’s challenge. Common sense cannot be a guide in responding to Thrasymachus because Thrasymachus has implied that what our common sense tells us about justice is a lie our oppressors manufacture. The Socratic elenchus proceeds by teasing out the contradictions in commonsense ideas, so it is of no use to us here. Instead, Plato has Socrates launch on extended speeches, pausing only for the occasional response from Glaucon or Adeimantus, so that Socrates can explore ideas that are far removed from the commonsense notions debated in the earlier dialogues.

Plato’s Theory of Forms is the most important bulwark against relativists such as Thrasymachus. Thrasymachus essentially argues for a “might makes right” position, such that truth and justice are nothing more than what the strongest people say they are. Plato responds that Thrasymachus and his like see everything as relative only because they are stuck in the “world of sights and sounds” that makes up our sensory experience. This world is not the real world but rather a shadow of the truly real world of Forms in which nothing changes, nothing passes away, and nothing is imperfect. Instances of justice in the visible world may be relative, and what seems just to one person may seem unjust to another, but the Form of Justice itself is absolute and incontrovertible. Thrasymachus’s relativism, then, is simply a consequence of not seeing the whole picture, like someone fixated on a rotten banana insisting that all bananas are brown.

The distinction Plato draws between the visible world and the intelligible world claims a separate and superior domain for abstract thought above concrete thought. Everything we can see and hear, he suggests, isn’t what is most real. What is most real is what we can grasp by means of the intellect. This includes not only mathematics but also the Forms that lie behind the visible world. Our knowledge of the visible world is imperfect and changing, so it amounts at best to true belief. The abstract principles that govern the intelligible world, however, are perfect and unchanging, and so they represent a higher form of knowledge than true belief. The metaphor of the line and especially of the cave are ingenious means of prompting his audience to consider that there is more to the world than mere appearance. Both metaphors suggest that we have an incomplete understanding of the world if we accept only what we see before us. Only a rational, searching mind can uncover the true nature of reality.

The Theory of Forms is perhaps not really a “theory,” since we find only compelling metaphors, not arguments, to persuade us of it. When Socrates first introduces the idea that behind the world of appearances are immaterial, eternal, and unchanging Forms, Glaucon and Adeimantus agree without further discussion. The most we get are the related metaphors of the sun, the line, and the cave, which combine to give a very compelling account of why we ought to believe Forms exist. Considering that the Theory of Forms is central to the argument of the Republic, the fact that Plato feels no need to argue for it suggests intellectual laziness. However, we may be misguided in viewing the discussion of Forms as a theory that needs to be argued for. Plato uses metaphors rather than arguments in support of Forms, which suggests he is not trying to persuade us of a particular point so much as trying to shift our way of looking at things.

In the Republic, the existence of Forms is not a conclusion we must reach but a premise we must start from. Plato never defines the Form of the Good, calling it instead an “unhypothetical first principle.” A “first principle” is the place at which a chain of reasoning begins. For example, if I reason, “there’s no car in the driveway, so my parents must be out, so the house must be locked, so I’d better look under the mat for the key,” the observation, “there’s no car in the driveway” is the first principle. If I were to say, “I’d better look under the mat for the key,” someone could ask “why?” and I could reply, “because the house is locked,” and someone could again ask, “why?” and I could reply “because my parents are out,” and so on. “There’s no car in the driveway” is a “hypothetical” first principle, because we are making a hypothesis in assuming that it’s true. If we are feeling philosophical, we can question this hypothesis by questioning whether our eyes tell us what’s real. Plato claims that answering this question leads us to posit the Forms that exist behind appearances, and positing these Forms will ultimately lead us to the Form of the Good. The Form of the Good itself is an unhypothetical first principle because it is not justified by any further facts or evidence. It is the one thing that is true and real in and of itself. As such, the existence of the Form of the Good, and Forms generally, is not something to be argued for. Rather, according to Plato, only by virtue of the Form of the Good can arguments hold ground at all. Without the Form of the Good, there would be nothing to justify any of our reasoning, so to demand reasons for why we should believe in the Form of the Good puts the cart before the horse.

The idea of a tripartite soul explains both the fact of inner conflict and the necessity for honing our reason. The idea that the soul is not simple but rather made up of three distinct parts is an ingenious solution to the problematic fact that we experience inner conflict: we can fight urges, want to want things, surrender ashamedly to temptation, and so on. This fact suggests that we have more than one set of drives working within us, and Plato’s theory of a tripartite soul is the first in a long string of psychological theories that lead down to Freud and beyond. By dividing the soul into a rational part, a spirited part, and an appetitive part, Plato also argues that our shameful or vicious actions are a consequence of giving into our baser desires. A virtuous person always follows the lead of reason, with spirit and appetite on a tight leash.

The Republic makes a number of recommendations in favor of authoritarian or even totalitarian government, and commentators have been sharply divided over how to interpret it on this score. Socrates’ ideal republic allows for limited personal freedoms and social mobility, is staunchly antidemocratic, and uses strict censorship and propaganda, to the extent of banishing all poets from the city. The philosopher Karl Popper has gone so far as to accuse the Republic as being the seminal influence behind the twentieth-century totalitarian regimes of Stalin and Hitler. Others have rightly pointed out that the Republic is the first sustained and rigorous examination of political philosophy in the Western tradition and that modern liberal democracy owes Plato a great intellectual debt. No simple answer exists to the question of whether the Republic’s political philosophy is benign or dangerous because the Republic itself is no simple book. We must recall that at least one purpose of the Republic is to provoke intense thought and discussion, so if we find passages shocking, we can assume this is what Plato would have wanted.