Chat with N. David Lewis

Philosopher and Modal Logician

About N. David Lewis

In the early 1980s, while most modal logicians treated possible worlds as formal devices, N. David Lewis insisted they were *real*, concrete, spatiotemporally isolated universes, each as actual to its inhabitants as ours is to us. His radical ontological commitment wasn’t metaphor or heuristic; it was the only way he saw to ground counterfactual truth conditions without ad hoc primitives. When critics called it 'incredible', he replied not with retreat but with precision: a full semantics for 'if it had rained, the match would have been canceled' that required no mysterious accessibility relations, just the closest world where it rained and the match’s causal structure held. His 1973 book Counterfactuals didn’t just refine a tool; it reoriented metaphysics toward systematic, logic-driven realism. He wrote in dense, dry prose, avoided conferences, and corresponded by hand, but his arguments forced every subsequent theorist on modality to choose: either build on his concrete pluralism or dismantle it brick by brick.

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Conversation Starters

Not sure where to begin? Try asking N. David Lewis:

  • “How do you respond to the charge that modal realism makes ethics trivial across worlds?”
  • “If worlds are concrete, what prevents transworld identity from collapsing into mere similarity?”
  • “What motivated your rejection of ersatzism, especially Stalnaker's propositional approach?”
  • “Did your analysis of 'might' counterfactuals (e.g., 'It might have been that p') require revising your 1973 semantics?”

Frequently Asked Questions

Did Lewis ever revise his stance on modal realism after the 1986 postscript to On the Plurality of Worlds?
No—he reaffirmed and refined it. The postscript addressed objections like the 'incredulous stare' and quantum branching, arguing that theoretical utility and explanatory power outweigh intuitive resistance. He tightened the account of world-boundness and clarified how counterpart theory avoids transworld individuals without compromising de re modality.
Why did Lewis reject David Kaplan's two-dimensional semantics for indexicals?
He saw it as redundant machinery. For Lewis, indexicals like 'I' or 'now' are perfectly handled by relativizing truth to centered worlds—worlds with designated agents and times—not by layering semantic dimensions. He argued Kaplan’s apparatus added complexity without resolving any genuine problem his own framework couldn’t handle more parsimoniously.
How does Lewis’s theory of properties as sets of possibilia interact with his nominalism?
It doesn’t—he rejected nominalism about properties. While he was a nominalist about *universals*, he treated properties as *sets* of concrete individuals across worlds, making them extensional, non-abstract entities. This allowed him to avoid Platonic forms while preserving property talk as legitimate set-theoretic discourse.
What role did Lewis assign to causation in his counterfactual analysis, versus earlier Humean accounts?
He reversed the dependency: instead of defining causation via counterfactuals, he defined counterfactuals via *causal dependence* among events in worlds. His 1986 paper 'Postscripts to 'Counterfactuals'' explicitly grounded the similarity metric on shared causal histories—not just structural resemblance—making causation primitive in the semantics.

Topics

metaphysicspossible worldslogic

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