 Lewis was also open to the program of analysing naturalness in terms of tropes—“particularised property instances” like Socrates’ paleness or Antony’s love of Cleopatra (see the entry on  tropes).  Lewis suggests that given a “sparse” theory of tropes (which would deny the existence of tropes like Socrates’s being either pale or flushed) and a modal realist ontology, one could define a perfectly natural property as one such that, for some maximal set of duplicate tropes, its instances are exactly the things that instantiate some member of that set (in the sense of ‘instantiate’ on which Socrates instantiates Socrates’ paleness). To avoid circularity, ‘duplicate’ here had better not be analysed in terms of sharing of perfectly natural properties. By contrast with the case of universals, it is not so easy for opponents of Lewisian modal realism to take over this account. The most straightforward approach would replace the dyadic ‘duplicate’ with a four-place predicate ‘\(t_1\) in \(w_1\) is a duplicate of \(t_2\) in \(w_2\)’; but treating this as primitive seems even more unsatisfactory than treating the dyadic ‘duplicate’ as primitive.