The word ‘sin’ is unlikely to be found in the index of a book on moral philosophy. It belongs to the vocabulary of theology. But the serious student of both subjects is bound to wonder how the concept of sin is to be related to the topics that interest moral philosophers. The problem is complicated by the evident fact that ‘sin’ is what W. B. Gallie has called an ‘essentially contested concept’ and that unanimity is as rare among moral philosophers as it is among theologians. It is not a matter, therefore, of applying an agreed philosophical method to a clearly defined theological concept, but of looking for a way of thinking about sin which is theologically defensible and which can approve itself to a reasonably sympathetic moral philosopher.