I recently picked up a copy of Plato’s Republic (OK, two actually), and at first glance, Plato’s just and unjust is not unlike the Buddha’s distinction between skillful and unskillful actions (kamma). Both seem like a middle way between, or possibly a synthesis of, Jeremy Bentham’s teleological utilitarianism and Immanuel Kant’s deontological categorical imperative.
That’s not to say that Bentham and Kant represent two ends of a single ethical spectrum, only that Plato and the Buddha take what Bentham and Kant stress and emphasis them together. With Plato and the Buddha, just/skillful actions aren’t simply judged to be just/skillful based upon their consequences, but also because there’s something inherently just/skillful about the actions themselves. In Buddhism, this would be due to the quality of the intentions behind the actions, and I think a similar principle applies in the Republic as well, although Plato would obviously say that it’s because they share in the form of Justice, or even of the Good.