Plato's longest and most famous dialogue, the Republic, begins with a simple question: what is justice, and is the just person happier than the unjust one? In Book II, Glaucon tells the story of the Ring of Gyges, which makes its wearer invisible. If you could act unseen, would you still be just? Glaucon suggests most people would not, and that justice is only a bargain the weak make to protect themselves. Socrates spends the rest of the work answering him.

The city as a large soul

Socrates proposes to look for justice first in a city, where it is “written in larger letters,” and then in the individual. He builds an imagined ideal city with three classes:

• Rulers (the guardians proper), who are wise and govern;
• Auxiliaries, soldiers who are courageous and defend the city;
• Producers, farmers, craftsmen and traders who supply its needs.

The city is just when each class does its own work and does not meddle in the work of the others. To keep the order stable, Plato proposes a “noble lie” or founding myth: citizens are told that the god mixed gold into rulers, silver into auxiliaries, and bronze and iron into producers. Children are placed according to their nature, not their parents' class.

The tripartite soul

Matching the city, Plato divides the soul into three parts: reason, which loves truth and calculates what is best; spirit, the seat of anger, pride and the sense of honor; and appetite, the desires for food, drink, sex and money. He argues for the division by noticing inner conflict: a thirsty person may want to drink yet hold back, so two parts must be at work. A just person is one in whom reason rules, with spirit as its ally, and appetite is kept in order. Justice is therefore a kind of inner health and harmony, and Plato concludes that it makes us happier than injustice, which is a sickness of the soul.

Philosopher rulers

Who should rule? Plato's startling answer appears in Book V. In Jowett's translation: “Until philosophers are kings, or the kings and princes of this world have the spirit and power of philosophy,” cities will have no rest from their troubles. Only those who know the Form of the Good can govern wisely. The guardian classes live without private property or private families, so that they have no interests apart from the city's good. Plato also argued that women could be guardians on the same terms as men, a radical idea in his time, though he still thought women weaker on average.

Plato was deeply critical of democracy. In Book VI he compares the city to a ship whose sailors quarrel over the helm, though none has learned navigation, while the true navigator is dismissed as a stargazer. In Book VIII he describes how democracy, by prizing freedom above all, slides into disorder and finally into tyranny.

Critics of the ideal city

The ideal city has had many critics. Aristotle argued that holding property and families in common would make people care less, not more: what belongs to everyone is looked after by no one. Modern liberal critics, writing after the dictatorships of the twentieth century, attacked the Republic as a blueprint for a closed society, with censorship of poetry, a ruling caste and an official lie. The philosopher Karl Popper made this charge famous in the 1940s. Defenders reply that Plato's city is a thought experiment about the soul as much as a political plan, and Plato himself, in the later Laws, sketched a more practical city governed by written law and a mixed constitution. Either way, the Republic forces every reader to ask who should rule and why.

Worked example: a quarrel inside one person

Theo, a student, has an exam tomorrow. A friend texts to invite him to a party. Theo feels three pulls at once:

• He wants the fun, music and food of the party. That is appetite.
• He also feels a flash of pride: he does not want to be the one who failed the exam, and he is annoyed with himself for even considering going. That is spirit.
• Calmly, he works out that an hour of study and a full night's sleep will serve him best, and that he can see his friends on Saturday. That is reason.

In Plato's terms, if Theo stays home, reason has ruled with spirit as its ally, keeping appetite in its place. If he goes to the party and regrets it, appetite has overruled reason, which Plato would see as a small disorder in the soul. Plato's claim is that a person whose soul is ordered in this way is not only more successful but happier, since he is not at war with himself.

Now scale up, as the Republic does. Suppose a city's merchants (appetite) control the government and pass whatever laws make them richest, while the soldiers (spirit) serve whoever pays them. Plato would call that city unjust in the same way.

A critic can push back on both pictures. Do people really have three distinct parts, or many shifting desires? And must the “reasoning” part of a city be a separate class of rulers, or can ordinary citizens deliberate together? This second question leads straight to the debate about democracy.