The problem I am going to discuss here concerns practical rationality, rationality not in thought but in action. More particularly, I am going to discuss the rationality, or absence of rationality (even, as one might put it, the contra-rationality or irrationality) of moral action. And ‘moral action’ shall mean here something done by someone who (let us suppose rightly) believes that to act otherwise would be contrary to, say, justice or charity; or again not done because it is thought that it would be unjust or uncharitable to do it. The question is whether in so acting, or refusing to act, this person will be acting rationally, even in cases where he or she believes that not only desire but self-interest would argue in favour of the wrongdoing.