From The Independent 23 October 2001 DAVID LEWIS (1941-2001) The death of David Lewis at the age of 60 has deprived philosophy of one of its most original and brilliant thinkers. Lewis was a systematic philosopher in a traditional sense, who created a system of thought (or metaphysical system) which attempts to reconcile the insights of modern science with pervasive elements of commonsense belief. Lewis was not a populariser and he had little to do with the more concrete and practical areas of philosophy. His work is forbiddingly abstract, and deals with many of the deepest and most difficult of philosophy's traditional concerns, including the nature of mind, causation, necessity and being. His general outlook is realist, in the sense of insisting on a clear distinction between the world and our knowledge of it, and materialist, in the sense of emphasising the fundamentally physical nature of our world. In these respects, his work does not differ from that of many contemporary philosophers; but what is distinctive of Lewis's contribution is the formidable rigour and attention to detail with which he pursued these metaphysical projects, and the imagination and brilliance with which he re-invigorated the study of metaphysics in the last few decades of the twentieth century. The originality and range of Lewis's work is remarkable. He once wrote that he 'should have liked to be a piecemeal, unsystematic philosopher, offering independent proposals on a variety of topics'. But, he added, 'it was not to be.' Certainly he did write on a vast range of areas – metaphysics, the theory of knowledge, the philosophy of language, mind and science – but his views in these areas were not all independent of each other. As he later put it himself, his thinking tended to converge on an idea he called 'Humean supervenience': that reality consists fundamentally of 'a vast mosaic of local matters of particular fact, just one little thing then another'. Everything else depends ('supervenes') on these 'little things': arrangements of physical qualities. Like the Enlightenment philosopher David Hume (hence 'Humean') Lewis denies that our world contains any necessity – anything that must be the case – other than that determined by physical law and mathematics. The basic idea of Humean supervenience can be illustrated by an image Lewis himself used: imagine a picture created out of tiny dots, which seen from a distance creates the impression of shapes. The shapes are completely determined by the dots, and in this sense they are nothing over and above the dots. In an analogous way, our world is completely determined by the nature of the physical qualities arranged in space and time. This somewhat austere vision of reality is supplemented in Lewis's work by subtle and penetrating accounts of the everyday phenomena of language and mind (the 'shapes', so to speak). His first book, the prize-winning Convention: a Philosophical Study (1969), gave the most rigorous account to date of what a convention is, and what it means to say language is conventional. The classic Counterfactuals (1973) brilliantly defended a way of understanding the claims we make about what might have been, or what would have happened if certain other things had not (that is, what is contrary to fact or 'counterfactual'). Lewis's theory here appealed to what was to become the most notorious aspect of his philosophy: his claim that other possibilities (or 'possible worlds') are as real as our actual world. Lewis went on to defend this claim in his John Locke lectures at Oxford University, published as On the Plurality of Worlds (1986). He defended the real existence of possible worlds in the same way that a mathematician might defend the real existence of numbers: their existence facilitates the task of giving an adequate account of the nature of our world. For example, in his theory of cause and effect, arguably the most significant advance in this debate since Hume, he appealed to possible worlds in explaining what it is for an effect to depend on its cause. However, Lewis found few converts to his view that there are other possible worlds. The view met with many 'incredulous stares' (Lewis's phrase) but few philosophers can honestly lay claim to a proper refutation of it. One of the most impressive aspects of his defence of the theory of possible worlds is the way in which Lewis follows the logic of the argument with remorseless consistency, and thus shows exactly how difficult it is for a consistent opponent to disagree with him. Professor Mark Johnston, the current chair of the Princeton Philosophy Department, has said that Lewis is 'the greatest systematic metaphysician since G.W. Leibniz'. The comparison is apt, not only because Leibniz invented this talk of possible worlds, but also because Lewis shares with Leibniz a commitment to the discovery of the true and ultimate consequences of his ideas, even if it leads to conflict with what is apparently obvious. Bertrand Russell once said that the philosopher must follow the wind of the argument wherever it leads; Lewis followed this advice with more intellectual honesty than most philosophers ever do. Despite the abstractness of the subject-matter, Lewis was justly famed for his writing style, at once elegant and informal. Strange as it may seem to an outsider, his books and essays can be read for pleasure, for their sheer intellectual brilliance, their clarity and their wit. David Lewis was born in Oberlin, Ohio in 1941. He studied at Swarthmore College as undergraduate and obtained his Ph.D. from Harvard University in 1967. His first academic appointment was at UCLA, and he taught there until moving to Princeton in 1970, where he remained until his death. He was a dedicated and muchloved teacher, well-known for the care and rigorous criticism which he brought to his students' work. Mark Johnston comments that he was 'sweet and stern ... always unfailingly generous with his time and with positive philosophical suggestions, and ruthless with his criticism.' Somewhat eccentric in manner, Lewis was known among philosophers for his lack of small talk and his tendency to give accurate, detailed and literal answers to the most ordinary questions. Though somewhat disconcerting to new acquaintances, friends found this aspect of his behaviour part of his charm. He was a railway enthusiast and collected model trains; visitors to his model train layout sometimes observed that he had created a whole possible world in his own home. Lewis and his wife Stephanie felt particularly at home in Australia, where they had many philosophical friends. He left there an abiding intellectual legacy, a sign of which is the honorary degree he received from the University of Melbourne (he also received honorary doctorates from the Universities of York and Cambridge). Unpretentious and modest in his tastes, he enjoyed the informality, straightforward openness and good humour of Australian society, and would often be found singing Australian folksongs during the annual conference of the Australasian Association of Philosophy. He was a keen follower of Australian Rules football, and occasionally used his favourite team (Essendon of Melbourne) in humorous examples to illustrate philosophical points. Tim Crane University College London Photograph by D.H. Mellor, used with permission