The audio file is a recording of David Lewis's talk to the Philosophy Department at La Trobe University, Melbourne, Australia, with Tim Oakley's introduction, and subsequent discussion, which started at 5.30 pm on 23 July 1981. It is posted here by permission of Tim Oakley and Stephanie Lewis. The PDF is of the paper, developed from that talk, first published in the proceedings of the Russellian Society of the University of Sydney in 1988. The paper was subsequently republished in David Lewis, Papers in Metaphysics and Epistemology (Cambridge: Cambridge University Press, 1999), pp. 262–90, and is posted here by permission of Stephanie Lewis and the Russellian Society.