A man called Thrasymachus loses his temper in Book I and says what he actually thinks: justice is whatever serves whoever is strong enough to define it, and the just man is a mug who ends up worse off than the unjust one.

The other nine books are the answer, and the answer requires building a city, dividing the soul into three parts, describing four ways a society decays, and explaining the nature of knowledge.

What it is like to read

Ten books, around 90,000 words, and it is the most influential book in Western philosophy and not the place to start with Plato. Start with the Apology or the Symposium.

It is also much more varied than its reputation. Book I is a Socratic dialogue that ends in failure. The middle books are political construction. Books VI and VII are metaphysics. Books VIII and IX are something close to political psychology, and Book X ends with a myth about the afterlife.

If you read only one part, read Book I plus the start of II — Glaucon's challenge is the sharpest statement of the problem the whole book exists to answer.

The challenge, and the method

Glaucon refuses to let Socrates off with the easy answer. He restates Thrasymachus' position more strongly than Thrasymachus managed, and demands a proof that justice is worth having in itself — not for reputation, not for consequences. He asks Socrates to imagine a just man thought to be wicked and an unjust man thought to be virtuous, and to show that the first is still better off.

Socrates' move is an analogy: justice in a single soul is hard to see, so look at it written large in a city, then read the result back.

That analogy is load-bearing and is where most serious objections land. It gives the three classes — producers, guardians, rulers — matched to three parts of the soul: appetite, spirit and reason. Justice, in both, is each part doing its own work and the right part in charge.

The parts that scandalize

Book V is where readers stop being comfortable. Women in the guardian class on equal terms with men, which was radical. The abolition of family and private property among the guardians. And the claim that no city will be well governed until philosophers rule or rulers philosophize.

Books VI and VII supply the epistemology that claim needs: the Sun, the Divided Line, and the Cave, which is treated on allegory of the cave, and the doctrine of Forms behind it.

Books VIII and IX then run the machine in reverse, describing how a good constitution decays through timocracy, oligarchy, democracy and tyranny, with a corresponding soul for each. The tyrant, who can do anything, is shown to be the least free and most wretched man alive — and that portrait is the actual answer to Glaucon.

Key passages

  • 327a–328c — the opening scene in the Piraeus.
  • 338c — Thrasymachus: justice is the advantage of the stronger.
  • 357a–368c — Glaucon's challenge.
  • 473d — the philosopher-ruler thesis.
  • 506e–509c — the analogy of the Sun.
  • 509d–511e — the Divided Line.
  • 514a–520a — the Allegory of the Cave.
  • 595a–608b — the second critique of poetry.
  • 614a–621d — the Myth of Er.

Whether he means it

The most important interpretive question, and the page should not pretend it is settled.

On the political reading, this is a proposal, and its author is recommending censorship, eugenic breeding, a ruling caste and the abolition of the family. Popper read it this way and called him a totalitarian.

On the psychological reading, the city is a device. Socrates says he is building it in order to see justice in the soul more clearly, says explicitly at 592b that it may exist nowhere on earth, and the political detail is scaffolding for an argument about individual character.

The text supports both and Plato declines to settle it. What is not in doubt is that everything after it is a response to it — including Aristotle's, which starts by objecting.