StalnakerLewis

StalnakerLewis is a term used to describe the influence of Robert Stalnaker and David Lewis on the possible-worlds analysis of conditionals and related modal notions. Their work provides a formal framework for evaluating counterfactuals by comparing the actual world to nearby possible worlds, treating worlds as concrete alternatives for linguistic and philosophical discussions.

Robert Stalnaker developed an early nearest-world semantics for counterfactuals. In his account, a counterfactual “If A, then B” is true at a world w if B holds in the closest A-world to w, where “closest” encodes minimal change from the actual world while preserving as much of its facts as possible. This approach emphasizes how facts about the actual world constrain hypothetical scenarios and how small departures from reality shape truth conditions.

David Lewis expanded this program by introducing a rigorous similarity ordering on possible worlds and by allowing for sets of equally closest worlds when a unique closest world does not exist. He defended modal realism—the view that possible worlds are as real as the actual world—and used the framework to analyze counterfactuals, belief revision, and other modal notions. Lewis’s work also underpinned general theories of conditionals and their relation to probability and decision theory.

Together, the Stalnaker-Lewis tradition has been influential in philosophy of language, logic, and cognitive science. It remains central to debates over the interpretation of counterfactuals, the semantics of conditionals, and the nature of possible worlds, though many alternative accounts have emerged and scholars continue to scrutinize assumptions about similarity, closeness, and realism.