  None of the solutions considered in this section are without their problems. The possibilist recognizes a distinction between existing and actually existing that requires a bloated ontology. Plantinga's solution requires mysterious entities — individual essences that could exist unexemplified and thus that are individuated independently of the individuals that instantiate them. Linsky and Zalta's solution runs contrary to the robust intuition that what individuals there happen to be is a contingent matter and requires that ordinary existents could exist as nonconcrete objects. And the final solution requires a distinction between how matters stand in a possible world or from the perspective of that possible world and how matters stand at that world, which many find mysterious and claim to require an unpalatable form of realism about possible worlds. The account also requires complicating one's quantified modal logic, as it entails that there are theorems of classical quantificational logic (for example, ∃x(a=x)) that are contingent, requiring either a revision of classical quantificational logic, and a movement to a free logic, in which case those problematic classical theorems are no longer theorems, or a restriction on the standard Rule of Necessitation of standard modal logic, in which case there are contingent logical truths. (For a more detailed discussion of the issues raised in this section, see the entry on the  possibilism-actualism debate.)