 But singletons are deeply mysterious. The usual metaphors that are used to introduce sets, metaphors about combining or collecting or gathering multiple things into one are less than useless when it comes to understanding the relationship between a singleton and its member. In (1993d), Lewis settles for a structuralist understanding of singletons. He also says that he “argued (somewhat reluctantly) for a structuralist’ approach to the theory of singleton functions” in (1991a), though on page 54 of (1991a) he appears to offer qualified resistance to structuralism.