Editing David Lewis (philosopher) (section)

===Realism about possible worlds===
What made Lewis's views about counterfactuals controversial is that whereas Stalnaker treated possible worlds as imaginary entities, "made up" for the sake of theoretical convenience, Lewis adopted a position his formal account of counterfactuals did not commit him to, namely [[modal realism]]. On Lewis's formulation, when we speak of a world where I made the shot that in this world I missed, we are speaking of a world just as real as this one, and although we say that in that world I made the shot, more precisely it is not I but a ''[[counterpart theory|counterpart]]'' of mine who was successful.

Lewis had already proposed this view 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). The theory was widely considered implausible, but Lewis urged that it be taken seriously. Most often the idea that there exist infinitely many causally isolated universes, each as real as our own but different from it in some way, and that alluding to objects in this universe as necessary to explain what makes certain counterfactual statements true but not others, meets with what Lewis calls the "incredulous stare" (Lewis, ''On the Plurality of Worlds'', 2005, pp.&nbsp;135–137). He defends and elaborates his theory of extreme modal realism, while insisting that there is nothing extreme about it, in ''[[On the Plurality of Worlds]]'' (1986). Lewis acknowledges that his theory is contrary to common sense, but believes its advantages far outweigh this disadvantage, and that therefore we should not be hesitant to pay this price.

According to Lewis, "actual" is merely an indexical label we give a world when we are in it. Things are [[logical truth|necessarily true]] when they are true in all possible worlds. (Lewis is not the first to speak of possible worlds in this context. [[Gottfried Wilhelm Leibniz]] and [[C.I. Lewis]], for example, both speak of possible worlds as a way of thinking about possibility and necessity, and some of [[David Kaplan (philosopher)|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 we find ourselves is no realer than any other possible world.)