David Kellogg Lewis

Western Philosophy
Twentieth century philosophy
Name: David Kellogg Lewis
Birth: September 28, 1941 (Oberlin, Ohio)
Death: October 14, 2001 (Princeton, New Jersey)
School/tradition: Analytic
Main interests
Logic, language, metaphysics, epistemology, ethics
Notable ideas
Possible worlds, modal realism
Influences Influenced
Leibniz, Hume, Carnap, Ryle, Quine, Strawson

David Kellogg Lewis (September 28, 1941 – October 14, 2001) is considered by many philosophers and observers of philosophy to have been one of the leading analytic philosophers of the latter half of the twentieth century. Lewis taught briefly at UCLA, and then Princeton University for the remainder of his career, but is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years. He is most famous for his theory of modal realism, but also made ground-breaking contributions in philosophy of language, philosophy of mind, general metaphysics, epistemology, and philosophical logic.


While Lewis's best known and most controversial theory is that there exists an infinite number of concrete and causally isolated possible worlds, of which this is just one, he has made lasting contributions to a large number of fields of philosophy. Indeed, a significant part of contemporary analytic philosophical work betrays the extent of his impact on the field.

Early life and education

Lewis was born in Oberlin, Ohio, to John D. Lewis, a Professor of Government at Oberlin College, and Ruth Ewart Kellogg Lewis, a distinguished medieval historian. He was known later in life for his formidable intellect; this intelligence was already manifest during his years at Oberlin High School, when he attended college lectures in chemistry. He went on to Swarthmore College and spent a year at Oxford (1959-1960), where he was tutored by Iris Murdoch and attended lectures by Gilbert Ryle, H.P. Grice, P.F. Strawson, and J.L. Austin. It was his year at Oxford that played a seminal role in his decision to study philosophy, and which made him the quintessentially analytic philosopher that he would soon become. Lewis went on to receive his Ph.D from Harvard in 1967, where he studied under W.V.O. Quine, many of whose views he came to repudiate. It was there that his connection with Australia was first established, when he took a seminar with J.J.C. Smart, a leading Australian philosopher. "I taught David Lewis," Smart would say in later years, "Or rather, he taught me."

Early work on convention

Lewis's first monograph was Convention: A Philosophical Study (1969), which is based on his doctoral dissertation and uses concepts of game theory to analyze the nature of social conventions. Lewis claimed that social conventions—such as the convention in most states that one drives on the right side of the road (not to the left), the convention that the original caller will re-call if a phone conversation is interrupted, etc.—are solutions to so-called "co-ordination problems." Co-ordination problems were, at the time of Lewis's book, a much under-discussed kind of game-theoretical problem; most of the game-theoretical discussion had circulated around problems where the participants are in conflict, such as the prisoner's dilemma.

Co-ordination problems are problematic, for, though the participants have common interests, there are several solutions. Sometimes, one of the solutions may be "salient," a concept invented by the game-theorist Thomas Schelling (by whom Lewis was much inspired). For example, a co-ordination problem that has the form of a meeting may have a salient solution if there is only one possible spot to meet at in town. But in most cases, people must rely on what Lewis calls "precedent" in order to get a salient solution. If both participants know that a particular co-ordination problem, say "which side should we drive on?" has been solved in the same way numerous times before, both know that both know this, both know that both know that both know this, etc.—this particular state Lewis calls common knowledge, and it has since been much discussed by philosophers and game theorists—then they will easily solve the problem. That they have solved the problem successfully will be seen by even more people, and thus the convention will spread in society. A convention is thus a behavioral regularity that sustains itself because it serves the interests of everyone involved. Another important feature of a convention is that a convention could be entirely different; people could just as well drive on the left (as is done in England and Japan); it is more or less arbitrary that people drive on the right (in America).

Lewis's main goal in the book, however, wasn't simply to provide an account of convention, but rather to investigate the "platitude that language is ruled by convention" (Convention, p. 1). The last two chapters of the book make the case that the use of a language in a population consists of conventions of truthfulness and trust among members of the population. Lewis recasts in this framework notions as those of truth and analyticity, claiming that they are better understood as relations between sentences and a language, rather than as properties of sentences.

Work on counterfactuals and modal realism

David K. Lewis in Australia, where he liked to visit.

Counterfactual conditional statements cause a problem for the standard logic of conditionals—statements of the form, "If p, then q"—because the standard logic of conditionals regards any conditional to be true in which the antecedent (the p statement) is false. But in counterfactual conditionals, the antecedent is false by definition because it is contrary to fact. Does this mean that all counterfactual statements are true? That conclusion is highly counterintuitive and is almost universally regarded as being untenable. For example, the statement, "If this copper penny had been in my pocket last July 1 [it was not in my pocket on that day], it would have been silver," hardly seems to be true, whereas the statement "If I had hit this drinking glass with a hammer last July 1 [I did not hit it then], it would have broken" seems to be true.

In Counterfactuals (1973), Lewis presented an analysis of counterfactual conditionals in terms of the theory of possible worlds. He had already proposed that solution in some of his earlier papers: "Counterpart Theory and Quantified Modal Logic" (1968), "Anselm and Actuality" (1970), and "Counterparts of Persons and their Bodies" (1971). In spite of significant technical advantages promised by this approach, the theory was widely considered to be too implausible to be taken literally, as Lewis urged it should be. Most often the idea that there exists an infinite number of causally isolated universes, each as real as this one, but different from it in some way, and that furthermore alluding to objects in this universe is necessary in order to explain what makes certain counterfactual statements true but not others, meets with what Lewis calls "a blank stare." Lewis defended and elaborated his theory of extreme modal realism, while insisting that there is nothing extreme about it, in On the Plurality of Worlds (1986). Lewis acknowledged that his theory is contrary to common sense, but believed that its advantages far outweigh this disadvantage, and that therefore people should not be hesitant to pay this price.

According to Lewis, what makes a counterfactual conditional of the form, "Had I made that shot our team would have won the game" true is that there is a world, as concrete as this one and significantly similar to it, in which one's counterpart makes rather than misses the shot and the counterpart of one's team wins the game. Had there been a world even more similar to ours in which my counterpart makes the shot but the counterpart of our team still loses the counterfactual would have been false. When people speak of counterfactual possibilities, they speak of what is the case in some possible world or worlds. Actuality, according to Lewis, is merely an indexical label we give to a world when we locate ourselves in it. Things are necessarily true when they are true in all possible worlds.

It should be noted that Lewis was not the first one to speak of possible worlds in this context. Leibniz and C. I. Lewis, for example, both spoke of possible worlds as a way of thinking about possibility and necessity, and some of David Kaplan's early work is on the counterpart theory. Lewis's original suggestion was that all possible worlds are equally concrete, and the world in which people find themselves is no more real than any other possible world.

This theory has faced a number of criticisms. In particular, it is not clear how people could know what goes on in other worlds. After all, they are causally disconnected from this; no one can look into them to see what is going on there. A related objection is that, while people are concerned with what they could have done, they are not concerned with what some people in other worlds, no matter how similar to them, do. As Saul Kripke once put it, a presidential candidate could not care less whether someone else, in another world, wins an election, but with whether he himself could have won it (Kripke 1980, p. 45). A more basic criticism is that introducing so many entities into our ontology violates the maxim of Occam's razor, which argues not to multiply theoretical entities beyond what is necessary to explain the facts theories aim to explain. One of the main concerns of Lewis (1986) is to address some of these criticisms.

Possible worlds are employed in the work of Robert Stalnaker (Inquiry, 1984), Saul Kripke (Naming and Necesity, 1980), Gideon Rosen ("Modal Fictionalism," 1990) and many others, but not in the concrete sense propounded by Lewis. None of these approaches has found anything near universal acceptance; few philosophers have found the price of extreme modal realism worth paying.

Other work

Lewis's final monograph, Parts of Classes (1991) sketched a reduction of set theory to mereology. It is worth remarking that Lewis became dissatisfied with parts of the theory presented in this book very soon after its publication, and it is currently out of print. He also published five volumes of collected papers: Philosophical Papers, Volume I (1983), which includes his early work on the counterpart theory, philosophy of language, and philosophy of mind, Philosophical Papers, Volume II (1986), which includes his work on counterfactuals, causation, and decision theory (it also contains a preface in which Lewis discusses Humean Supervenience, the name he gave to his overarching philosophical project), Papers in Philosophical Logic (1998), Papers in Metaphysics and Epistemology (1999), two of the papers in this volume ("Elusive Knowledge" and "Naming the Colours") have been reprinted in the Philosopher's Annual as among the 10 best papers for the years in which they had originally appeared, Papers in Ethics and Social Philosophy.

Together, these volumes collect some 99 papers—almost all of the papers published during Lewis's lifetime. Among Lewis's most important theories discussed therein are the counterfactual theory of causation, the concept of semantic score, a contextualist analysis of knowledge, and a dispositional theory of value.

At Princeton, Lewis was a gifted mentor of young philosophers, and trained dozens of successful figures in the field, including several current Princeton faculty, as well as faculty now teaching at most of the leading programs in the U.S. Among his most prominent students were David Velleman, Peter Railton, and Wayne A. Davis. His influence, direct and indirect, is evident in the work of many prominent philosophers of the current generation.

Later life and death

Lewis suffered from severe diabetes for much of his life, which eventually grew worse and led into kidney failure. In July of 2000, he received a kidney transplant from his wife, Stephanie. The transplant allowed him to work and travel for another year, before he died suddenly and unexpectedly from further complications of his diabetes, on October 14, 2001.

Since his death, a number of posthumous papers have been published, on topics ranging from truth and causation to philosophy of physics. Lewisian Themes, a collection of papers on his philosophy, was published in 2004.


  • Kripke, Saul A. Naming and Necessity. Cambridge, MA: Harvard University Press, 1980. ISBN 0674598458
  • Lewis, David K. Convention: A Philosophical Study. Cambridge, MA: Harvard University Press, 1969.
  • Lewis, David K. Counterfactuals. Oxford: Blackwell Publishers, 2001. ISBN 0631224955
  • Lewis, David K. On the Plurality of Worlds. Malden, Mass.: Blackwell Publishers, 2001. ISBN 0631224963
  • Lewis, David K. Papers in Ethics and Social Philosophy. New York: Cambridge University Press, 2000. ISBN 0521582490
  • Lewis, David K. Papers in Metaphysics and Epistemology. New York: Cambridge University Press, 1999. ISBN 0521582482
  • Lewis, David K. Papers in Philosophical Logic. New York: Cambridge University Press, 1998. ISBN 0521582474
  • Lewis, David K. Philosophical Papers, Vol. I. New York: Oxford University Press, 1983. ISBN 0195032039
  • Lewis, David K. Philosophical Papers, Vol. II. New York: Oxford University Press, 1986.
  • Stalnaker, Robert C. Inquiry. Cambridge, Mass.: MIT Press, 1984. ISBN 0262192330


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