 For attempts to reply to the argument from within the framework of possible worlds semantics, see among other places Stalnaker (1984) and Yalcin (2018); for discussion of a related approach to semantics which aims to avoid these problems, see  situations in natural language semantics (see entry).  Another option is to invoke impossible as well as possible worlds; one might then treat propositions as sets of worlds, which may or may not be possible. If there is an impossible world in which there are only finitely many primes but in which 2+2=4, that would promise to give us the resources to distinguish between the set of worlds in which  (13)  is true and the set of worlds in which  (14)  is true, and hence to explain the difference in truth-value between  (15) and (16).  For an overview of issues involving impossible worlds, see Nolan (2013).