Modal Realism

Modal realism is the metaphysical thesis, most closely associated with philosopher David Lewis, that possible worlds are concrete, spatiotemporally unified realities — not abstract objects, fictional constructions, or ersatz representations. Every way a world could possibly be is, according to modal realism, a way some world actually is. These worlds are causally and spatiotemporally isolated from one another, which explains why we cannot observe them directly.

The thesis is radical because it multiplies ontology extravagantly: if there are infinitely many possible ways things could be, there are infinitely many actual worlds. Critics charge that this violates Occam's razor — the principle that entities should not be multiplied without necessity. Defenders reply that theoretical utility, not ontological parsimony, is the proper criterion: modal realism provides the cleanest semantics for modal logic and the most straightforward analysis of counterfactual conditionals.

The systems-theoretic analogue is instructive. In formal verification and state space analysis, engineers routinely reason about all possible configurations of a system, even though only one configuration is actual at any moment. Modal realism treats this reasoning practice not as a useful fiction but as ontological truth: the unactualized possibilities are as real as the actualized one, differing only in their causal connection to the observer. Whether this is philosophical insight or philosophical excess remains one of the most productive debates in contemporary metaphysics.