If you think that there is not merely an order to the universe to be discerned by man, but a correct order, that must be understood to be obeyed (in the sense of conformity to what is best, not in the sense of submission to a rule), then justice becomes a highly relevant topic. Indeed, it is elevated to the level of a cornerstone concept in the organization of any good society. Plato and Aristotle were systematic thinkers. This means, they tried to build a complete philosophy in which every part coherently fits into every other part. Thus, justice must make metaphysical sense, as much as it does moral and political sense. For both Plato and Aristotle, that means reconciling both their metaphysical, and their moral and political notions, with the challenge of Parmenides.