 However, Lewis’s version of realism is by no means the only conception of possible worlds. According to an influential set of rival accounts, possible worlds, although real entities, are not concrete ‘other universes’, as in Lewis’s theory, but abstract objects such as (maximal) possible states of affairs or ‘ways the world might have been’. (See Plantinga 1974; Stalnaker 1976; van Inwagen 1985; Divers 2002; Melia 2003; Stalnaker 1995; also the separate entry on Possible Worlds. A state of affairs S is ‘maximal’ just in case, for any state of affairs S*, either it is impossible for both S and S* to obtain, or it is impossible for S to obtain without S*: the point of the restriction to the maximal is just that a possible world should be a possible state of affairs that is, in a relevant sense, complete.)