QuestionsWhat is a just society?Justice is each part of the city and of the soul doing its own task
What is a just society?Virtue and harmony

Justice is each part of the city and of the soul doing its own task

Justice in the city and in the soul

Plato c. 428 BC · c. 348 BCBeyond appearances, the eternal Forms.
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What it claims

Justice is inner harmony, the health of the soul; and the unjust person has a sick soul.

Why

A city is just when each group does its own task; the soul, which has three parts, is just in the same way.

In short

"Justice is each part of the city and of the soul doing its own task" is an argument by Plato (Republic IV, 433a-444e (c. 375 BC)) on the question "What is a just society?" (answer: virtue and harmony). Its claim: Justice is inner harmony, the health of the soul; and the unjust person has a sick soul. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P6). AskPhilosophy presents 3 objections, 1 of them with a documented reply, and the original text behind each premise, checked word for word.

Republic IV, 433a-444e (c. 375 BC)
Valid with an implicit premise

In Book II, Glaucon challenges Socrates: is justice merely a convention we put up with out of fear (see the ring of Gyges and Thrasymachus), or a good in itself? To answer, Socrates looks for justice "writ large" in an imaginary city, because it is easier to read there than in a single soul. He distinguishes three groups (those who produce, those who defend, those who govern) and finds that the city is just when each does its own task. Then he finds the same three levels in the soul: desire, spirit and reason. For Plato, justice is therefore a harmony, in the city as in the soul, and injustice a disorder that makes the soul sick. That is why this argument sits in the "harmony" camp: justice is neither a contract, nor a right, nor equality, but the good order of the whole. The P1..P6 → C form is our reconstruction of Book IV.

P1A city is just when each of its three groups (producers, guardians, rulers) does the task for which its nature is best suited, without meddling in the others'.
P2A person is just in the same way as the city: the soul also has three parts (desire, spirit, reason), and it is just when each does its own task.
P3In the soul, doing one's task means: reason, which is wise and watches over the whole soul, rules; spirit obeys it and is its ally.
P4When each part is in its place, a person masters himself, becomes one being instead of many, and tunes his three parts like the notes of a scale: this is a harmony.
P5When the order breaks down, one part revolts against the whole and wants to rule in its place: the soul is in a kind of civil war, and this order contrary to nature is a disease, just as health is the natural order of the elements of the body.
P6 · implicit premiseThere is no other harmony of the soul than justice: a soul at one with itself is necessarily just.
Therefore justice is inner harmony, the health of the soul; and the unjust person has a sick soul.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.
Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P6, the computer finds a case where the soul is in harmony without being just (a perfectly organised, self-assured gang leader) and where, not being just, it is nevertheless not sick.
P1, P2, P3 and P4 form a chain: just city, so just soul, so reason rules, so harmony. P5 says that without this order there is disease. But to conclude that the unjust soul is sick, we need P6, which Plato assumes without stating it as such: if harmony were possible without justice, an unjust soul could be in good health. The real debate is about P2 (are the city and the soul built alike?), P1 (who decides natures?) and P6 (is a well-organised tyrant not in harmony?). Plato answers the last one in Books VIII and IX, by describing the tyrant's soul as the most disordered of all.
Analogy. An orchestra: if each instrument plays its own part and listens to the others, the conductor can lead and the music is right. If the drum drowns everyone or the violin refuses to follow, you hear at once that the orchestra is sick.

Objections

3

Each objection is a conversation: click to see who replies and where the debate stands.

Karl Popper 1945 · attacks premise 1

For Popper, Plato's "justice" is not equality before the law: it is keeping everyone in his place in his class, in the service of the state's stability. He reads in it a "totalitarian" programme: the individual is subordinated to the city, and the governed have no rights against those who govern (The Open Society and Its Enemies, vol. I, ch. 6, "Totalitarian Justice").

a defender replies
No reply possible: objection raised in 1945, after the author's death (-348).
R
Ronald B. Levinson defender of the argument · 1953
Levinson answers Popper in a whole book. He argues that Popper reads Plato through the totalitarian regimes of the twentieth century, strains some texts and ignores those that go against his thesis (In Defense of Plato).
Where the debate stands (our summary)Both sides agree on one point: the city Plato describes is strictly hierarchical. The core disagreement is how to read it: a real political programme, as Popper thinks, or a large image used to understand the soul, as Plato says at the start (II, 368d-369a). An open point, with no widely accepted reading. Our summary.
Aristotle (critique of the city's unity) 335 BC · attacks premise 2

Aristotle blames Socrates for wanting to make the city as much of a unity as possible. If unity is pushed too far, the city ceases to be a city: it is by nature a multitude, and it would become a family, then a single individual. Even if a lawgiver could do it, he should not, for he would destroy the city (Politics II, 2). He mainly targets the sharing of wives, children and property among the guardians (Republic V), but the ideal of a city as unified as one man is what the comparison in P2 rests on.

no reply
No reply possible: objection raised in -335, after the author's death (-348).
The debate goes on (our summary) Plato would answer (our summary) that the harmony he describes does not abolish plurality: the just person tunes his three parts "like the three terms of the musical scale", he does not melt them into one (IV, 443d-e). Aristotle replies that a city's unity must come from exchange between different beings, not from sharing everything. The disagreement is about how much unity a city can bear.
John Rawls 1971 · attacks premise 1

In Plato, everyone's place depends on his "nature", hence on his talents. For Rawls, natural talents are distributed by chance: from a moral point of view, no one deserves to be born gifted, or to be born into a favourable background. A just society must therefore give everyone equal chances of reaching positions, and arrange its rules so that they also benefit the worst off (A Theory of Justice, § 12 and § 17).

no reply
see the argument
No reply possible: objection raised in 1971, after the author's death (-348).
The debate goes on (our summary) Plato does not lock everyone into his birth class: he provides that a child of bronze may be born of gold parents, or the reverse, and that the rulers move him to another rank (III, 415b-c). Rawls would answer that recognising talent is not enough to say that it gives a right to a better place or to greater advantages. Our summary.
Where the debate stands (our summary)Plato and Rawls agree that places should not rest on birth. They differ on what talent gives: for Plato, a task to fulfil for the good of all; for Rawls, no particular merit, hence no advantage that does not also benefit the worst off. A core disagreement, with no widely accepted answer.
What today's voices say
Criticize it
Karl PopperThe Open Society and Its Enemies attacks Plato's ideal city as a forerunner of closed, totalitarian societies.