Explanatory Structures: Modal Realism and other Necessitarian Systems

When David Lewis put forward modal realism—the view that all possible worlds are concrete—he neglected the explanatory dimension of such a model of existence. In this paper my goal is to put modal realism in the context of the principle of sufficient reason (PSR) and consider the explanatory structure of a modal realist model of existence. I consider two modal realisms—the platonist and the theistic modal realism. According to the former, the explanans for the existence of all possible worlds is the principle of plenitude according to which all possible worlds must exist. According to the latter, the explanans for the existence of all worlds is God. The two modal realisms converge in the fact that the explanans for any existing particular fact is constituted by the entire logical space. This is the main place where modal realisms governed by the PSR diverge from traditional necessitarian systems in which the explanans for any particular facts is constituted only by its own world. Also, the point of divergence between the two modal realisms is twofold. First, the platonist version doesn’t admit God in its existence. Second, it allows for what “brute facts” which in the end are not brute given that they have explanation in terms of the principle of plenitude.