QuestionsWhat is a just society?Justice is only a useful pact
What is a just society?The contract

Justice is only a useful pact

Epicurus on justice

Épicure 341 BC · 270 BCHe rejects the fate of the natural philosophers: some things happen by necessity, others by chance, others through our own agency.
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What it claims

Justice is exactly a useful pact: it varies with places and circumstances, and a law that stops being useful stops being just.

Why

Justice does not exist in itself: it is an agreement not to harm one another, and what stops serving that agreement stops being just.

The same idea through history
Plato375 BCIn the Republic, Glaucon sets out, without endorsing it, an origin of justice as a compromise between people. It is the same starting point, but he draws the opposite conclusion: one would be just only out of weakness.Thomas Hobbes1651Hobbes also makes justice arise from a covenant, but under the authority of a sovereign who enforces it.
In short

"Justice is only a useful pact" is an argument by Épicure (Principal Doctrines, 31-37) on the question "What is a just society?" (answer: the contract). Its claim: Justice is exactly a useful pact: it varies with places and circumstances, and a law that stops being useful stops being just. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 3 premises. AskPhilosophy presents 3 objections, 3 of them with a documented reply, and the original text behind each premise, checked word for word.

Principal Doctrines, 31-37
Valid

For Epicurus there is no Justice written in the sky or in the nature of things. There are people who, to live in safety, promise not to harm one another. That pact is what we call justice. It can differ from one country to another, because what is useful changes. And a law that no longer serves this common safety is no longer just, even if it stays a law. Animals, which cannot promise anything, have neither justice nor injustice.

P1Justice does not exist in itself: it exists only where people have made an agreement neither to harm nor be harmed.
P2Whatever is just is useful to people's dealings with one another: natural justice is the sign of what serves not harming one another.
P3What the law lays down and proves useful to people's dealings with one another is just, whether or not it is the same everywhere.
Therefore justice is exactly a useful pact: it varies with places and circumstances, and a law that stops being useful stops being just.

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.
The argument is valid. If being just requires a pact (P1) and being useful (P2), and if a useful pact is enough to be just (P3), then being just is having a useful pact. The objections target P1 (there would be a natural justice) and P2/P3 (usefulness cannot ground justice).
Analogy. A highway code. Driving on the right is not just in itself: it is a rule we chose so as not to collide. It differs from country to country, and a rule that no longer prevents accidents gets changed. Epicurus thinks of justice this way, but for the whole of common life.

Objections

3

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

Aristotle 330 BC · attacks premise 1

There are two kinds of justice in the city: a natural one, which has the same force everywhere and does not depend on what people think, and a legal one, which is indifferent at first but counts once the law is laid down. Some believe all justice is of the second kind, because it changes from place to place. This is only partly true: even what is just by nature can change.

Épicure replies
E
Epicurus (Principal Doctrines 36) 300 BC
Epicurus does not name Aristotle, but his maxim 36 answers on this point. In general, justice is the same for all: it is what is useful in dealings between people. Only its application varies with countries and circumstances. What stays constant is the usefulness of not harming one another, not a Justice independent of people.
Where the debate stands (our summary)Our summary: the dispute is about what stays constant under the variations. Aristotle sees a human nature that makes some things just in themselves; Epicurus sees only a shared need for safety. Both accept that laws change.
Cicero (On the Laws) 52 BC · attacks premise 3

It is absurd to think that whatever the laws of a people decide is just. Would the laws of the thirty tyrants of Athens have been just because they were called laws? And if justice is measured by utility, whoever thinks an injustice will profit him will commit it if he can: justice would no longer exist, one interest destroying another.

Épicure replies
No reply possible: objection raised in -52, after the author's death (-270).
E
Epicurus (Principal Doctrines 37 and 35) 300 BC · Epicurus (earlier text)
Epicurus does not say every law is just. On the contrary, he says a law that does not serve people's dealings with one another is no longer just. A tyrant's law, which harms those who live under it, is therefore unjust. As for whoever secretly breaks the pact, he can never be sure of not being discovered, and that fear is what makes injustice bad for him.
Where the debate stands (our summary)Our summary: Cicero criticises a crude utilitarianism (what benefits me is just). Epicurus speaks of a common usefulness, that of all members of the pact. The remaining difficulty is whether common usefulness can explain a duty when nobody is watching.
Glaucon, in Plato (the ring of Gyges) 375 BC · attacks the conclusion

If the just man and the unjust man were both given the power never to be seen, we would find them going the same way, driven by the desire to have more. So the pact is respected only out of weakness, and the stronger would break it.

Épicure replies
see the argument
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Epicurus (Principal Doctrines 34-35) 300 BC
Injustice is not bad in itself, but through its consequences: the fear of punishment. And whoever secretly breaks the pact can never be sure of staying hidden, even if he has escaped ten thousand times. A ring that makes one invisible for certain does not exist; without it, the unjust man lives in anxiety.
David Hume (the sensible knave) 1751
Hume admits the force of the objection. In the case of justice, a sensible knave may think that a particular injustice, without really breaking the social union, will pay off. "Honesty is the best policy" is a good general rule, but it has exceptions. He then replies that the knave is often betrayed by his own maxims and loses all trust.
Where the debate stands (our summary)Our summary: still an open question. Epicurus and Hume reply that injustice almost never pays in the long run. Their critics reply that "almost never" is not enough to ground a duty: one perfectly hidden case would be enough to overturn the thesis.