Ersatzism

Ersatzism is the view that possible worlds are abstract representations or surrogates — ersatz constructions — rather than concrete realities. The term was coined by David Lewis as a dismissive label for any theory of possible worlds that rejects his modal realism. Ersatzers hold that our world is the only concrete universe, and that talk of other possible worlds is talk about abstract entities of some kind: maximally consistent sets of propositions, fictional stories, structural descriptions, or combinations of actual properties.

The ersatzer's project is to provide a semantics for modal language without multiplying concrete existents. The leading versions include linguistic ersatzism (possible worlds are maximally consistent sets of sentences in some actual or idealized language), pictorial ersatzism (worlds are structural descriptions), and magical ersatzism (worlds are primitive abstracta whose nature is not further analyzable). Each faces the representation problem: if possible worlds are abstract, how do they represent concrete possibilities without smuggling in modal notions?

Lewis argued that all ersatz theories are either circular (they use modal notions to define their surrogates) or inadequate (they cannot capture all genuine possibilities). The ersatzer's response is that modal realism's advantage is purchased at an unacceptable ontological cost, and that the technical problems of ersatzism are soluble with sufficient ingenuity.

Ersatzism is the philosophy of the engineer who builds models rather than the explorer who claims territories. Its weakness is not its ontology but its confidence: ersatzers have been trying to solve the representation problem for half a century, and the solutions keep requiring either more modal primitives or more abstract machinery. The honest conclusion is that ersatzism works well enough for most applications — logic, linguistics, computer science — but fails as a foundational metaphysics. This is not a failure of the approach. It is a sign that possible worlds semantics was never meant to be a metaphysics in the first place.