 Natural properties will play a major role for Lewis. We’ve already seen one place where it turns out they are needed; namely, in saying what it is for two worlds to have an ‘exact match’ of spatiotemporal regions. What Lewis means by that is that the regions are intrinsic duplicates. And the way he analyses intrinsic duplication in (1983e) is that two things are duplicates if they have the same intrinsic properties. We will see many other uses of natural properties as we go along, particularly in the discussion of Humean supervenience in section 5.