The Price of Possibility: Realism, Structure, and the Limits of Ontological Commitment

 

The Price of Possibility: Realism, Structure, and the Limits of Ontological Commitment

The history of modal metaphysics is often framed as a struggle between explanatory elegance and ontological economy. Since Aristotle distinguished between energeia (actuality) and dynamis (potentiality), philosophers have sought a rigorous account of what it means for something to be possible. In the contemporary era, this quest reached its zenith in David Lewis’s modal realism, which posits that possible worlds are concrete realities no less genuine than our own. While Lewis’s framework provides a powerful semantic and metaphysical engine for modal logic, it incurs a massive ontological cost: the acceptance of an infinite plurality of causally isolated universes. The resulting debate, involving thinkers such as D. M. Armstrong and Kris McDaniel, centers on whether modal realism requires Lewisian concreteness or whether possibility can be grounded in actual structures. I argue that while Lewisian modal realism offers unmatched explanatory unity, his commitment to strict isolation and counterpart theory is an unnecessary burden. Alternatives such as McDaniel’s structured-world frameworks and Armstrong’s states-of-affairs ontology provide a more plausible realism by grounding possibility in actual structure rather than ontological excess, without collapsing into mere ersatzism.

Lewis’s modal realism begins with the principle that possible worlds are not abstract representations but concrete totalities. Our world is not metaphysically privileged; "actual" is an indexical term, analogous to "here" or "now." Consequently, modal claims are analyzed through quantification across this plurality: a proposition is possible if it holds in at least one world and necessary if it holds in all (Lewis 2). This achieves extraordinary explanatory success, providing a concrete interpretation of modal logic and counterfactuals. However, this success requires a "reductive" program that replaces trans-world identity with counterpart theory. Because worlds are strictly isolated, individuals cannot exist in multiple worlds; rather, there exists a counterpart who resembles the original (Lewis 194). This move preserves modal isolation but strikes many as a violation of ordinary modal intuition, as it effectively suggests that when I say, "I could have become a musician," I am speaking of someone else entirely.

Kris McDaniel’s Modal Realisms emerges as a critical intervention, arguing that the features of Lewis’s system are not essential to the realist project itself. McDaniel argues that Lewis’s commitments—world-bound individuals, strict isolation, and counterpart theory—are a choice, not a necessity. By exploring models where objects may literally overlap across worlds, McDaniel restores a more intuitive understanding of trans-world identity. As McDaniel notes, if we permit overlap, we move closer to ordinary linguistic practice, where we genuinely refer to the same individual across modal scenarios (McDaniel 134). However, this flexibility creates a new problem: if objects can exist in multiple worlds, the boundaries of those worlds lose the absolute ontological independence that Lewis relied upon to maintain his system’s elegance.

This tension between Lewis’s unity and McDaniel’s plausibility points toward an alternative tradition represented by D. M. Armstrong. Armstrong’s ontology prioritizes actual existence, positing that reality consists of particulars instantiating universals, with states of affairs serving as the truthmakers for our modal claims (Armstrong 115). For Armstrong, possible worlds are not concrete universes but maximal combinations of states of affairs governed by combinatorial principles. This approach grounds possibility in the structure of actual reality rather than in a distant plurality. The distinction between Lewis and Armstrong is thus a fundamental methodological schism: Lewis seeks to explain possibility through ontological expansion, whereas Armstrong and McDaniel seek to explain it through structural analysis.

Critics of structured-world ontologies often argue that if worlds are not concrete universes, they must be mere abstract representations, thus collapsing realism into a form of ersatzism. This objection rests on a category mistake: it equates "reality" with "spatiotemporal concreteness." Structured worlds are not merely symbolic maps; they are constitutive grounds. They function as the territory of possibility itself, grounding modal truths and constraining counterfactual reasoning through their internal relations. Nothing in the definition of realism requires that all real entities resemble ordinary physical objects. Therefore, structured-world frameworks preserve the realist commitment to non-actual entities without demanding an infinite plurality of universes.

Ultimately, the choice between Lewis and the structured-realist tradition reflects a deeper philosophical divide. Lewis prioritizes systematic explanatory power at the cost of ontological extravagance, famously responding to the "incredulous stare" of his critics by insisting that theory determines ontology (Lewis 133). In contrast, structured-realist frameworks like those of Armstrong and McDaniel prioritize metaphysical plausibility and ontological discipline, following the medieval dictum that entia non sunt multiplicanda praeter necessitatem (entities should not be multiplied beyond necessity). While Lewis remains the more unified architect, McDaniel and Armstrong prove the more plausible metaphysicians. A successful realism must bridge this gap, grounding the reality of possibility in genuine structures rather than in the baroque proliferation of entities. The challenge remains to explain how possibility can be understood without multiplying being beyond the requirements of explanation.

Works Cited

Armstrong, D. M. A World of States of Affairs. Cambridge UP, 1997.

Lewis, David. On the Plurality of Worlds. Blackwell, 1986.

McDaniel, Kris. "Modal Realisms." Philosophical Studies, vol. 125, no. 2, 2005, pp. 131-152.