 This topic, natural properties, was one of very few topics where Lewis had a serious change of view over the course of his career. Of course, Lewis changed the details of many of his views, in response to criticism and further thought. But the idea that some properties could be natural, could make for objective similarity, in ways that most sets of possibilia do not, is notably absent from his writings before “New Work”. Indeed, as late as “Individuation by Acquaintance and by Stipulation” (1983c), he was rather dismissive of the idea. But natural properties came to play central roles in his metaphysics and, as we see here, his theory of mind. As Lewis notes in “New Work”, much of the impetus for his change of view came from discussions with D. M. Armstrong, and from the arguments in favour of universals that Armstrong presented in his (1978).