This paper is about the rationality of moral action, and so about a problem that is as old as Plato but which still haunts moral philosophy today. It is about the rationality of following morality; of refraining from murder or robbery for instance, and being faithful in keeping contracts and promises, even where this seems to be against our interest and contrary to what we most desire. The problem of the rationality of morality arises most obviously over such actions and (...) therefore has to do particularly with the virtue of justice, because it is here that self-interest and morality often seem to clash. Then reason may represent itself as on the side of self-interest and the fulfilment of present desire; so unless it can be shown that acting justly is a necessary part of practical rationality, cynics like Thrasymachus will always say that there is no good reason to pass up an advantage for the sake of acting justly, and plenty of reason not to pass it up. (shrink)