 Introductions to our topic often emphasise a connection to similarity. Whereas any two objects, no matter how similar or dissimilar, share infinitely many properties and are divided by infinitely many properties, sharing of natural properties “makes for similarity” (Lewis  1983,  347;  1986b,  60), while division by natural properties “makes for dissimilarity”. These vague generalizations suggest two projects: that of analysing naturalness in terms of similarity, and that of analysing similarity in terms of naturalness.