What is the function of moral principles within the body of moral knowledge? And what must be the nature of moral principles in order for them to carry out this function? A specific set of answers to these questions is widely accepted among moral philosophers – so widely accepted as almost to constitute a sort of orthodoxy. The answers embody a view of the place of principles within the body of morality which crosses the lines between cognitivism and non-cognitivism. Though I have put the question in cognitivist terms and shall discuss it in those terms, I think a similar question and a more or less parallel discussion could be given in non-cognitivist terms. Perhaps the time-honoured debate between the two positions can be suspended, at least temporarily, while we examine, not the nature of morality, but its structure.