 To appreciate Plantinga’s account, note first that a challenge to actualism that we have not focused on hitherto is the nature of  possible worlds  themselves. The main reason for this is simply that the basic semantic argument for possibilia laid out in the Introduction to this entry does not assume or require them. If, however, one takes possible world semantics of some ilk seriously, so that possibility and necessity are correlated in some manner with truth at some or all possible worlds, then, insofar as possibilia are said to exist in non-actual possible worlds, such worlds are themselves reasonably classified as some variety of possibilia themselves, some variety of merely possible, non-existent object. Hence, because Plantinga takes possible world semantics seriously, the first task he sets for himself is to define possible worlds in an actualistically acceptable way.