Languages are functions from sentences to meanings, and the language of a population is the one in which they have a convention of truthfulness and trust.
In metaphysics and modal logic, Lewis is known principally for ‘Counterpart Theory and Quantified Modal Logic’ (1968) and On the Plurality of Worlds (1986). Based on its theoretical benefits, Lewis argues for modal realism: other possible worlds and the objects in them are just as real as the actual world and its inhabitants. Lewis develops a non-standard form of modal logic in which objects exist in at most one possible world, and for which the necessity of identity fails. Properties are identified with the set of objects that have them in any possible world, and propositions as the set of worlds in which they are true. He also develops a finergrained concept of structured properties and propositions.
In philosophical logic and philosophy of science, Lewis is best known for Counterfactuals (1973), ‘Causation’ (1973), and ‘Probabilities of Conditionals and Conditional Probabilities’ (1976). He developed a formal semantics for counterfactual conditionals that matches their truth conditions and logic much more adequately than the previously available material or strict conditional analyses. Roughly, a counterfactual is true if its consequent is true in every possible world in which its antecedent is true that is as similar overall to the actual world as the truth of the antecedent will allow. Lewis then defended an analysis of causation in terms of counterfactuals: c caused e if e would not have occurred if c had not occurred or if there is a chain of events leading from e to c each member of which is counterfactually dependent on the next. He presents a reductio ad absurdum argument to show that conditional probabilities could not be identified with the probabilities of any sort of conditional.
Lewis has also written on visual experience, events, holes, parts of classes, time travel, survival and identity, subjective and objective probability, desire as belief, attitudes de se, deontic logic, decision theory, the prisoner’s dilemma and the Newcomb problem, utilitarianism, dispositional theories of value, nuclear deterrence, punishment, and academic ethics.
See also CAUSATION, CONDITIONAL , FUNC- TIONALISM , MEANING , POSSIBLE WORLD.
W.A.D.