  Like concretism, abstractionism provides a reasonably clear and  intuitive account of what worlds are and what it is to exist in them, albeit from a decidedly different perspective. Although, as noted in   §2.2.2,  the fact that modality is a primitive in abstractionist definitions  of possible worlds arguably compromises its ability to provide semantically  illuminating truth conditions for the modal operators, those definitions can be taken to illuminate the connection between our basic modality concepts and the evocative notion of a possible world that serves as such a powerful conceptual tool for  constructing philosophical arguments and for analyzing and developing solutions to philosophical problems. In this regard, particularly  noteworthy are: Plantinga's (1974) influential work on   the ontological argument  and the free will defense against   the problem of evil;  Adams' (1974, 1981) work on   actualism  and actuality; and Stalnaker's (1968, 1987) work on   counterfactual conditionals  and   mental content.  