American philosopher. Lewis was born in Oberlin, Ohio, and educated at Swarthmore, at Oxford for a year, and at Harvard where he gained his doctorate in 1967. He taught at the university of California, Los Angeles from 1966 to 1970, after which he taught at Princeton. Lewis contributed to a wide range of issues, and in the second half of the twentieth century became the most influential metaphysician working in the analytic tradition. His first book, Convention: A Philosophical Study (1969) rehabilitated the notion of convention, at the time regarded with deep suspicion both by philosophers of language and by political theorists. Counterfactuals (1973) introduced the now classic possible worlds treatment of such statements. Lewis was remarkable for an uncompromising realism about the possible worlds in terms of which his analyses work, and much modern discussion has centred on ways of having the benefits of his accounts without incurring the metaphysical costs. Such attempts were rebuffed in The Plurality of Worlds (1986). Lewis addressed a wide variety of other issues in his Collected Papers (3 vols., 1983, 1986, 2000). His book Parts of Classes (1991) developed a mereological approach to set theory.