David Lewis

Quick Facts

• Name: David Lewis
• Full name: David Kellogg Lewis
• Lived: 1941-2001
• Born: Oberlin, Ohio
• Main fields: metaphysics, philosophy of language, logic, philosophy of mind, philosophy of science
• Academic home: Princeton University, where he taught from 1970 to 2001
• Best known for: modal realism, possible worlds, counterpart theory, counterfactuals, Humean supervenience, and convention theory

The Big Question

How can philosophers talk clearly about what is possible, necessary, causal, lawful, or conventional without treating those ideas as vague mysteries?

Lewis's answer was bold: take the logical machinery seriously. If possible worlds explain possibility, counterfactuals, meaning, causation, and laws better than the alternatives, then maybe possible worlds are not just shorthand. Maybe they are part of what exists.

In One Minute

David Lewis was one of the most important analytic philosophers of the late 20th century. He helped make analytic metaphysics systematic again: a connected theory of what reality must be like if our best explanations are true.

His most famous view is modal realism. This says that possible worlds are real concrete worlds, not just stories, descriptions, or useful diagrams. Our world is actual for us because it is the world we are in. A different world is actual for its inhabitants in the same indexical way that "here" means this place when we say it.

That sounds extravagant. Lewis knew it did. He defended it because it explained what could have happened, what must happen, and what would have happened if things had gone differently.

What They Taught

Lewis taught that philosophy should pay its ontological bills. "Ontology" means the inventory of what a theory says exists. If a theory talks about possible worlds, laws, causes, chances, meanings, and selves, then the philosopher should say what those things are and what the theory costs.

His main teaching starts with possible worlds. A possible world is a complete way things could be. Many philosophers use possible worlds as abstract models. Lewis went further. He said a possible world is a concrete reality, like our world, but spatiotemporally and causally isolated from it. No spaceship can travel there. No signal can cross over.

This is modal realism. "Modal" means having to do with possibility and necessity. On Lewis's view, "It is possible that there are blue swans" means that in at least one possible world, there are blue swans. "It is necessary that two plus two is four" means that in every possible world, two plus two is four. Modal claims become claims about what happens across the whole plurality of worlds.

Lewis used the same framework for counterfactuals. A counterfactual is an "if things had been different" claim: "If I had left earlier, I would have caught the train." Lewis says we test it by looking at the nearest possible worlds where I leave earlier. "Nearest" means most similar in the relevant respects, not physically nearby. If in those nearest worlds I catch the train, the counterfactual is true.

This counterfactual account also shaped his theory of causation. A cause is something that makes a difference. If the match had not been struck, it would not have lit. Hard cases remain, such as backup causes and overdetermination. But Lewis made the basic thought precise: causal dependence is a pattern of counterfactual dependence.

Lewis's other large picture is Humean supervenience. It says that the world is, at bottom, a vast mosaic of local matters of fact: particular qualities at particular points, arranged across space and time. Laws, chances, causes, and dispositions do not float above the world as extra governing powers. They supervene on the mosaic. "Supervene" means there cannot be a change in the higher-level facts unless something changes underneath.

His best system account of laws belongs here. A law of nature is a statement in the best true system for summarizing the whole mosaic. The best system balances simplicity and strength. A random list of every falling object is strong but not simple. "Things happen" is simple but not strong.

Lewis also explained conventions as stable solutions to coordination problems. Driving on the right works if others do it too. Driving on the left would also work if everyone expected it. A convention is the stable pattern sustained by mutual expectations.

Key Ideas With Examples

• Modal realism: possible worlds are real concrete worlds. Example: you could have been a surgeon because a real world contains a person very much like you who takes that path.

• Possible worlds: complete realities, not partial scenes. Example: a world where Napoleon wins at Waterloo also includes its weather, bodies, languages, planets, and consequences.

• Actuality: "actual" works like "here." Example: we call this world actual because we are in it; inhabitants of another world would call their world actual.

• Counterpart theory: you do not literally exist in other worlds. Example: "I could have been left-handed" is true if some close counterpart of you is left-handed.

• Counterfactuals: claims about what would happen if something else happened. Example: "If the glass had been dropped, it would have broken" is true if the closest worlds where it is dropped are worlds where it breaks.

• Causation: one event causes another when the second depends on the first in the right counterfactual pattern. Example: the thrown stone caused the window to break if the window does not break in the nearest worlds where the stone is not thrown.

• Humean supervenience: all facts depend on the total arrangement of local qualities across spacetime. Example: if two worlds match perfectly in every local physical detail, then they also match in their laws, chances, and causal facts.

• Convention: a regular social pattern kept in place by shared expectations. Example: a word means what it does because speakers expect one another to use it that way.

Major Works

• Convention: A Philosophical Study (1969): explains conventions through coordination problems, shared expectations, and regular behavior. Its examples include language, signaling, and social practices where people need to converge on the same pattern.

• Counterfactuals (1973): develops a possible-worlds semantics for "would have" claims. Counterfactuals are judged by the closest worlds in which the antecedent is true.

• Philosophical Papers, volumes 1 and 2 (1983, 1986): major essays on language, mind, time, causation, probability, and metaphysics.

• On the Plurality of Worlds (1986): Lewis's fullest defense of modal realism. It argues that concrete possible worlds earn their cost by explaining possibility, necessity, counterfactuals, properties, propositions, and meaning.

Why It Matters

Lewis matters because he made metaphysics precise, ambitious, and hard to ignore. Many philosophers reject modal realism. But they still have to answer his challenge: if possible worlds are only fictions, abstract objects, or useful language, can they explain as much?

He also reshaped analytic metaphysics. Earlier analytic philosophy often treated grand metaphysical systems with suspicion. Lewis showed how metaphysics could be technical without becoming empty. His work connected logic, language, science, mind, and ordinary modal talk into one large framework.

His style mattered too. He did not hide the oddness of his view. He argued that every theory has costs. The question is whether the payoff is worth them.

Proponents, Critics, and Opponents

Lewis belongs to Analytic Philosophy. He inherited W. V. O. Quine's demand for explicit ontology and theory choice, but he accepted modal commitments that Quine resisted. He also drew on the formal semantic tradition associated with Rudolf Carnap.

His Humean supervenience develops themes from David Hume: no hidden necessary connections above the pattern of facts. Saul Kripke is a major contrast. Kripke used possible worlds in modal logic but did not accept Lewis's concrete worlds or counterpart theory.

Robert Stalnaker, Alvin Plantinga, and others offered less costly accounts of possible worlds. Critics argue that modal realism is too ontologically expensive, counterpart theory strains "I could have done otherwise," and Humean laws are too weak because they summarize nature instead of governing it.

Lewis's influence is still deep. David Chalmers uses possible worlds and supervenience in arguments about consciousness and physicalism. Hilary Putnam resisted Lewis's confident metaphysical realism. Donald Davidson overlaps with Lewis on language and events, but Lewis gives convention, counterfactuals, and worlds a more explicit metaphysical structure.

Related Pages

• Analytic Philosophy
• W. V. O. Quine
• Rudolf Carnap
• David Hume
• Saul Kripke
• David Chalmers
• Hilary Putnam
• Donald Davidson