David Kellogg Lewis (1941–2001) was born in Oberlin, Ohio, of academic parents, earned a B.A. in philosophy at Swarthmore College, and an M.A. and Ph.D. at Harvard; during his postgraduate studies he worked for some time at the Hudson Institute, where he became interested in game theory and its potential for addressing conceptual problems. His Ph.D. dissertation became the book Convention: A Philosophical Study (Harvard University Press, 1969), a portion of which is reproduced here by permission. He taught at UCLA from 1966 to 1970 and then spent the remainder of his career at Princeton, also spending much time in Australia. His other writings include Counterfactuals (1973), On the Plurality of Worlds (1986), and Parts of Classes (1991). His numerous papers have been collected into five volumes.