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.

In starting out like this I shall be addressing the concerns of one whom I might label ‘the moral doubter’: one who has problems about the rationality of acting morally rather as many Christians have problems about the existence of evil in the world. This person wants to be convinced and may be particularly attached to morality, but has a worry about why ‘in the tight corner’ anyone has reason to do what there seems to be reason enough not to do, or again not to do what there seems reason enough to do. My moral worrier may not be in any doubt about what is right and wrong, and is therefore different from an immoralist such as Thrasymachus in Plato's Republic, who insists that justice is not a virtue but rather ‘silly good nature.’