David Lewis

⬅️ Hub: Philosophers (MOC) · Related: 60.9 Philosophy · Metaphysics & Mind (MOC)

David Lewis (1941–2001) built unusually systematic accounts of possibility, counterfactuals, causation, laws, mind, and language. The notebook focuses on three connected ideas: modal realism, a possible-world semantics for counterfactuals, and Humean supervenience.

Modal realism

Lewis took possible worlds to be concrete realities of the same general kind as the actual world. “Actual” functions indexically: it identifies the world we inhabit, much as “here” identifies a location, without granting it a unique mode of being.

• A proposition is possible if it is true at least at one world.
• It is necessary if it is true at every world.
• Individuals are world-bound; talk about what I might have been is analyzed through a sufficiently similar counterpart in another world, not a numerically identical person inhabiting two worlds.

The attraction is theoretical power: one ontology supplies a precise language for modality, counterfactual comparison, properties, propositions, and other analyses. The cost is ontological extravagance—the claim that innumerable unactualized worlds are as real as ours.

Counterfactuals and “closest” worlds

A counterfactual such as “If the match had been struck, it would have lit” is not evaluated by changing only one fact mechanically. Lewis compares worlds where the antecedent is true and asks whether the consequent holds in the most relevantly similar ones.

The notebook turns this into a practical reasoning discipline:

• Replace vague regret—“I could have”—with “What would have needed to be different?”
• Compare the actual outcome with the nearest plausible alternative, not a fantasy with every constraint removed.
• Identify a difference-maker: in the nearest worlds without X, does Y still occur?
• Distinguish alternatives requiring one small change from alternatives requiring a different person, history, or physical possibility.
• Judge decisions partly by the range of foreseeable alternatives, not only by the outcome luck produced.

Philosophical versus practical use

Lewis’s theory does not provide an algorithm for choosing “the closest” real-life scenario. Similarity depends on context and on which laws, background conditions, and facts are held fixed. The notebook’s decision method is inspired by the theory, not entailed by every detail of Lewis’s metaphysics.

Causation as difference-making

The intuitive test is:

In the nearest relevant world where the putative cause did not occur, would the effect still have occurred?

If not, the cause is counterfactually relevant. Real causal analysis is more difficult because of preemption, overdetermination, omissions, and changes that alter several downstream conditions at once. The counterfactual remains a powerful question even when a simple “but-for” test is insufficient.

Humean supervenience

The OCR reads “Human Supervenience,” but Lewis’s term is Humean supervenience. In simplified form, the world is a mosaic of local particular facts; everything else supervenes on their total arrangement. Laws, chances, and higher-level patterns are not additional governing entities floating above the mosaic.

Lewis combined this with a best-system account: the laws are the generalizations belonging to the system that best balances simplicity and informational strength. The project is not merely “look at concrete facts and ignore abstractions.” It is a substantive and controversial metaphysical reduction of modal, nomic, and other facts to the distribution of fundamental qualities and relations.

Modern physics creates pressure for some versions of the proposed supervenience base, a limitation Lewis recognized. The view should therefore be learned as a structured research program, not as a settled consequence of empiricism.

Notebook applications

1. Counter regret: What nearby change was actually available then?
2. Plan branches: Which present action makes a desirable future world more likely?
3. Diagnose cause: Which factor changes the outcome across otherwise similar cases?
4. Reduce fatalism: Actuality does not imply necessity.
5. Improve moral and legal judgment: Separate outcome, reasonable foresight, available alternatives, and causal contribution.

Sources

• Stanford Encyclopedia of Philosophy, “David Lewis”.
• Stanford Encyclopedia of Philosophy, “David Lewis’s Metaphysics”.

Handwritten Notebook Pages

Source

Open David Lewis notebook pages — start at notebook p. 8 (file not included in this release).

Retrieval questions

• In what sense is “actual” indexical for Lewis?
• Why does counterpart theory matter to modal realism?
• What makes one counterfactual comparison more relevantly similar than another?
• Why is “Human Supervenience” an OCR error, and what does Humean supervenience claim?
• Which causal cases make a simple but-for test inadequate?