Lewisian vs. Ersatz Modal Realism

1. The Case for Modal Realism: A Philosopher's Paradise?

David Lewis was well aware that his theory of a plurality of concrete worlds would be met with what he called "the incredulous stare." Why would a serious philosopher propose such an outlandish idea? His answer was simple: because the theory is immensely useful, or "serviceable." Lewis argues that the philosophical benefits of accepting his view are so great that they outweigh its initial implausibility.

The core motivation is to achieve a reductive analysis of modality. For a philosopher with a background in formal methods, this should be an appealing goal. Much as Tarski's semantics provided a clear, extensional account of logical truth, Lewis wanted to "explain away" the mysterious intensional notions of possibility and necessity in simpler, non-modal terms.

How does modal realism achieve this reduction? It transforms talk about modality into standard extensional quantification over a domain of objects (the worlds and their parts).

• "It is possible that $P$" becomes "There exists a world $w$ where $P$ is true."
• "It is necessary that $P$" becomes "For all worlds $w$, $P$ is true at $w$."

This move is powerful. It replaces the special modal operators $◊$ and $\square$ with the familiar quantifiers $\exists$ and $\forall$, whose logic is well understood. Modality becomes just another topic of scientific and philosophical inquiry, analyzed by quantifying over a particular kind of entity: worlds.

2. The Ersatzist Alternative: A More Sober Ontology

Most philosophers, while admiring the power of Lewis's system, cannot bring themselves to accept its ontological cost. The primary motivation for ersatzism is to gain the benefits of possible-worlds-talk without committing to a "bloated ontology" of concrete universes.

The ersatzist strategy is to claim that possible worlds are substitutes or replacements (the meaning of the German word Ersatz). They are not concrete realities, but actual, abstract objects that represent possibilities.

There are many ways to construct these abstract substitutes. You don't need to master the details of each, but it's important to recognize the common strategy.

The key takeaway is that all these views—whether they use sets of sentences, propositions, states of affairs, or combinatorial constructions—share two features:

1. Ontological Parsimony: They posit only one concrete world (the actual one) plus a domain of abstract objects. This is seen as more commonsensical than Lewis's view.
2. Representationalism: Other possible worlds are not possibilities themselves; they are abstract things that represent possibilities.

3. Comparison and Critique: The Price of Paradise

So, we have a clear trade-off: Lewis's modal realism offers great explanatory power at a high ontological cost. Ersatzism offers a cheaper ontology, but does it deliver the same explanatory goods? This is the crucial point of comparison. Lewis's main argument is that ersatzism, in all its forms, ultimately fails to provide a genuine, non-circular reduction of modality.

The Central Objection: Circularity

Ersatz theories seem to smuggle in the very modal notions they are supposed to be explaining.

• Sententialism defines a world as a maximally consistent set of sentences. But what does "consistent" mean? It means it's possible for all the sentences to be true together. The analysis is circular.
• Combinatorialism defines worlds as combinations of basic elements. But it must specify which combinations are possible. Armstrong's version, for instance, allows any combination, leading to the conclusion that Bertrand Russell could have been a poached egg. To avoid this, one needs a constraint on possible combinations, reintroducing modality.

On this point, Lewis claims a decisive advantage. His theory is not circular. He explains possibility by quantifying over a domain of concrete things (worlds). The explanation stops there. The ersatzer, by contrast, explains possibility in terms of abstract objects which themselves must be characterized in modal terms.

Further Objections to Ersatzism

Lewis raises other powerful objections that highlight the expressive limitations of building worlds from purely actual materials.

These objections put the ersatzer in a difficult position. To solve them, they often have to add more and more complex machinery to their theory, making it less simple and elegant, or they must concede that their theory cannot account for certain intuitive possibilities.

Conclusion

We have arrived at a classic philosophical impasse, a choice between two competing research programs, each with its own significant costs and benefits.

• David Lewis's Modal Realism offers a powerful, reductive, and unified theory of modality. It provides a "philosopher's paradise" of analytical tools. The price of entry, however, is a belief in an infinity of concrete universes, a belief that strains credulity and raises difficult epistemological questions.
• Ersatzist Alternatives offer a more sober and commonsensical ontology, requiring belief only in one concrete world and a realm of abstracta. The price, however, is that these theories seem unable to provide a genuine reduction of modality. They are often circular, relying on primitive, unanalyzed modal notions, and they face technical challenges in accounting for all the possibilities we might want to countenance.

The debate forces us to weigh what we value most in a philosophical theory: Ontological parsimony or ideological (explanatory) power? A sober inventory of what exists, or a powerful engine for analyzing complex concepts? There is no easy answer.

Key Takeaways:

• Modal Realism (Lewis): Justified by its "serviceability." It provides a non-circular, reductive analysis of modality by treating it as quantification over concrete, existing worlds. Its main cost is its extravagant ontology.
• Ersatzism: A family of views that use actual abstract objects (sets of sentences, propositions, etc.) as substitutes for worlds. Its main benefit is ontological sobriety.
• The Core Comparison: The central dispute is about reduction. Lewis argues that only his view truly explains modality in non-modal terms. He critiques ersatzism for being circular (relying on notions like "consistency" or "possibility") and for being unable to account for possibilities involving alien properties or indiscernibles.
• The Trade-Off: The choice represents a fundamental philosophical tension between ontological cost (what you must believe exists) and ideological cost (what concepts you must accept as primitive or unexplained).

Preview of the Next Lesson:
Having explored the deep metaphysical debate about what possible worlds are, we will now turn to see how this new way of thinking about modality, pioneered by Kripke and Lewis, had revolutionary consequences for other areas of philosophy. In the next lesson, we will evaluate Kripke's argument for the existence of necessary a posteriori truths and its impact on the analytic-synthetic distinction, a cornerstone of traditional epistemology and philosophy of language.