David Lewis

Early Life and Education

David Kellogg Lewis was born on September 28, 1941, in Oberlin, Ohio, to academic parents (his father was a professor of government, his mother a medieval historian). He studied philosophy at Swarthmore College before attending Harvard for graduate work, where he studied under W.V.O. Quine. His doctoral dissertation, completed in 1967, was supervised by Quine and became his first book, Convention (1969).

At Harvard, Lewis also came under the influence of Quine's colleagues and visitors, including Donald Davidson and Saul Kripke. Despite Quine's deep skepticism about modal logic and possible worlds, Lewis developed what would become the most robust philosophical defense of the reality of possible worlds.

Convention (1969)

Lewis's first book addressed a fundamental question in the philosophy of language and social theory: what is a convention, and how do conventions arise without explicit agreement? Drawing on game theory (particularly the work of Thomas Schelling), Lewis analyzed conventions as self-sustaining regularities in behavior: everyone conforms, expects others to conform, and prefers to conform given that others do. This analysis influenced linguistics, economics, and social philosophy.

Counterfactuals and Causation

Counterfactuals (1973) developed a systematic analysis of counterfactual conditionals using possible worlds. "If the match had been struck, it would have lit" is true if, in the nearest (most similar) possible worlds where the match is struck, it lights. Lewis developed a detailed account of the similarity relation between worlds and applied this framework to analyze causation: roughly, an event c causes an event e if, in the nearest possible worlds where c does not occur, e does not occur either.

This counterfactual analysis of causation became one of the most influential and debated theories of causation in contemporary philosophy, generating an enormous literature on preemption, overdetermination, and other problem cases.

Modal Realism

On the Plurality of Worlds (1986) presented Lewis's most controversial and celebrated thesis: modal realism. Lewis argued that possible worlds — the ways things might have been — are concrete, spatiotemporally isolated universes, as real as our own. The actual world is not ontologically privileged; "actual" is simply an indexical term, like "here," that picks out the world we happen to inhabit.

Modal realism provides a uniform analysis of modality: a proposition is possible if it is true at some world, necessary if true at all worlds, contingent if true at some but not all. Lewis argued that the theoretical benefits of modal realism — its ability to provide reductive analyses of modality, counterfactuals, properties, propositions, and more — outweigh the initial incredulity it provokes.

Philosophy of Mind and Materialism

In the philosophy of mind, Lewis defended a version of functionalism combined with the identity theory: mental states are defined by their functional roles (their causal relations to inputs, outputs, and other mental states), and in humans these functional roles are realized by brain states. His "Mad Pain and Martian Pain" (1980) illustrated the flexibility of this approach: a being with very different physical constitution (Martian) might realize pain functionally, while a human with unusual reactions (Mad) might have the right physical state but not the typical functional role.

Lewis taught at UCLA (1966–1970) and Princeton University (1970–2001). He died on October 14, 2001, in Princeton, New Jersey, at age 60. His range across metaphysics, philosophy of language, philosophy of mind, epistemology, philosophy of science, and ethics was virtually unmatched among his contemporaries.