My title advertizes a paradox. The characteristic complaint of the sceptic is that others make assumptions they are not entitled to make. A philosophical sceptic is committed to a systematic refusal to accept such assumptions in the absence of the kind of justification they think is required. A sceptic who, none the less, helps himself to such an assumption, seems to be caught in a paradoxical position. This is the kind of situation in which, it seems, certain eighteenth-century sceptical philosophers were placed in relation to the ‘principle’ of natural order. They did not doubt that there is such a principle, that there is a source or ultimate cause of the order to be found in the universe. And yet, on their own terms, is not the existence of such a principle something we should expect them to have doubted? What I shall try to do in this lecture is to bring out why they did not doubt the existence of such a principle and how serious their failure to do so is for their sceptical position.