 The argument as such is hardly convincing. Firstly, one author’s modus ponens is another’s modus tollens. Some have used similar considerations to argue against Lewis’s modal realism (see Skyrms 1976; Naylor 1986): if one believes in possible worlds (of the Lewisian kind) as ways things could have been, then by parity of reasoning one should believe in impossible worlds (ditto) as ways things could not have been. But impossible worlds are too much to swallow, so (by modus tollens) one should not believe in Lewis’s modal realism.