"Justice" is the word philosophers reach for when asking what we owe each other — politically, economically, between strangers, across generations. Plato's Republic is the founding text in the Western tradition; he asked what a just city would look like, and what a just soul would look like, on the suspicion that the answers had to be the same.
Modern political philosophy is dominated by a Plato-vs-Hobbes choice. The Platonic side asks what's intrinsically right and tries to build institutions that match. The Hobbesian side asks what arrangements rational self-interested people would consent to. John Rawls's Theory of Justice is the most influential twentieth-century version of the consent-based approach: imagine you didn't know whether you'd be born rich or poor, talented or not, healthy or sick (the "veil of ignorance") — what social arrangement would you choose?
Rawls argued you'd pick a society where inequalities are only justified if they make the worst-off better off. Robert Nozick countered that focusing on outcomes (who has what at the end) ignores how things were acquired (was the process just?) — what matters is whether transactions were voluntary. Communitarians (MacIntyre, Sandel) argued both sides forget that justice is embedded in particular communities and traditions, not derivable from a thin universal premise.
The argument is alive because the stakes are real: every redistributive policy, every property claim, every reparations debate is a fight about which theory wins.