This is the most political question in philosophy — and the least answerable. Plato answered through order. Locke, through rights. Rousseau, through equality. Kant, through reason. Marx, through economics. Every answer is convincing. Every one of them, once put into practice, breeds a new injustice. That does not mean the question is pointless. It means the question is alive.
In the Republic Plato builds the ideal just city. Justice is when each man does his own work: philosophers rule, warriors defend, craftsmen produce. Each minds his own business — and does not meddle in anyone else's. It sounds reasonable — until you notice: who decides whose work is whose? Plato's answer: the philosophers. Who appointed themselves. The first answer to the question of justice already contains the problem inside it.
Locke wrote this in 1689 — after the English revolution, when Europe was working out what lawful power is. His answer: every human being has natural rights — to life, to liberty, to property. The state exists only to protect them. If it violates them, the people have the right to rise. This became the foundation of liberalism, of the American Declaration of Independence, and of modern human rights. That thought changed the world more than any army did.
Rousseau asked the uncomfortable question: what if the right of property is itself an injustice? "The first man who, having enclosed a piece of ground, bethought himself of saying this is mine, was the founder of civil society." And all our miseries came from there. Rousseau did not want to abolish property — he wanted equality of opportunity. But his language turned out to be destructive: inequality as the source of injustice — that idea set off two centuries of revolutions.
The categorical imperative: act only in such a way that you could will your action to become a universal law. This is not a formula for happiness — it is a formula for justice. What is just is what can be made a principle for everyone. Kant strips away emotions, interests, consequences. Pure reason is what remains. It is beautiful — and cold. In real life reason always acts in somebody's interest.
Marx turned the question over: not "what is justice?" but "who benefits from defining it that way?". Law, morality, philosophy — all of it is a superstructure over the economic base. The capitalist pays the worker less than the worker produces — that is exploitation, and no "right to property" will justify it. Marx did not build a theory of justice — he exposed everyone else's. That is his strength. And his weakness.
If every age has its own justice — is there anything that stays just always?