 In the first chapter of Plurality Lewis takes a much more orthodox Quinean line. He argues, at great length, that the best version of many philosophical theories requires quantification over possibilities. In traditional terms, he offers an extended  indispensability argument  for unactualised possibilities. But traditional terms are perhaps misleading here. Lewis does not say that possibilities are absolutely indispensable, only that they make our philosophical theories so much better that we have sufficient reason to accept them.