The more I reread The Republic, the more I think that the guy who argued with Socrates at the beginning had a solid point — justice is whatever the rulers say. Incidentally, this argument leads straight to anarchy (the system which is taken to be bad, without any arguments).

Well, he said, have you never heard that forms of government differ; there are tyrannies, and there are democracies, and there are aristocracies?

Yes, I know.

And the government is the ruling power in each state?

Certainly.

And the different forms of government make laws democratical, aristocratical, tyrannical, with a view to their several interests; and these laws, which are made by them for their own interests, are the justice which they deliver to their subjects, and him who transgresses them they punish as a breaker of the law, and unjust. And that is what I mean when I say that in all states there is the same principle of justice, which is the interest of the government; and as the government must be supposed to have power, the only reasonable conclusion is, that everywhere there is one principle of justice, which is the interest of the stronger.