American philosopher. While he had an important impact on many areas of philosophy, and areas outside philosophy such as linguistics and game theory, it is perhaps in metaphysics that he was the most influential.
Lewis's materialist metaphysics were connected by a common thread (see physicalism/materialism). Lewis defended “Humean Supervenience”: the doctrine that “all there is to the world is a vast mosaic of local, particular matters of fact, just one little thing and another” (Lewis, 1986b, ix). Once the facts about the instantiation of intrinsic properties (see extrinsic/intrinsic) at each space–time point have been determined, all the contingent truths about the world have been. Ordinary objects are aggregates of point/instant-sized entities spread out in space and in time, for example. Lewis was not committed to the truth of Humean supervenience, but he did want to argue that while it might be overturned because of findings in physics, there were not any good philosophical reasons to reject it.
Lewis defended a doctrine of TEMPORAL PARTS to explain objects' existence through time: I can be sitting now and not-sitting later because I have one temporal part now that is bent in the right way and another, different, temporal part later that is not. Lewis called the challenge of explaining how objects could have opposite intrinsic properties at different times, while still remaining the same object, the problem of temporary intrinsics. Lewis defended this doctrine of perdurance as offering the best solution to the problem of temporary intrinsics as well as to puzzles about IDENTITY across time.
A key component of Lewis's later metaphysical work was his reliance on an objective distinction between natural properties, properties that make for objective resemblance and “carve nature at its joints,” and abundant properties that correspond to any arbitrary collection of possible objects whatsoever. Lewis used natural properties not only in his proposed solutions to puzzles about properties, but also in his account of laws and causation, in his characterization of his materialism, and his metaphysics of thought and language.
Lewis defended theories of laws of nature (see law of nature), CAUSATION (see the extended essay), dispositions (see disposition) and chance that were compatible with his Humean supervenience doctrine. Laws of nature are not extra facts over and above the particular goings-on in the world, but are rather determined by those facts: Lewis defended a regularity theory of laws of nature. Objective chances (e.g., of radioactive decay) were analyzed in terms of laws–they corresponded to the chances stated by these laws of nature. Dispositions and causation were both given analyses in terms counterfactual (see counterfactuals) conditionals, of what would have happened if other things had happened (e.g., the disposition of salt to dissolve is a matter, in part, of what would have happened if the salt were put in water), which was in turn analyzed in terms of what possibilities were most similar to what actually occurred.
One of the areas Lewis is most famous for is his work on what possibilities are, and in general the metaphysics of MODALITY AND POSSIBLE WORLDS (see the extended essay). Lewis argued that ordinary talk about necessity and possibility is best analyzed as involving quantification over possible worlds: to say that something is necessary is to say it is true in all worlds, to say it is possible is to say it is true in at least one. Lewis's account of these possible worlds was distinctive: he argued they were best understood as entities maximally connected by spatio-temporal relations, or by relations analogous to spatio-temporal ones. Worlds, in other words, were entities just like the actual world that we are all a part of, and exist in just the same way. This understanding of possible worlds became known as modal realism (see especially Lewis, 1986a).
Lewis also suggested a new approach to dealing with de re modal claims, that is, claims about what is necessary or possible for an object. Lewis suggested that we analyze what is possible for an object in terms of what similar objects, called counterparts, that object has in other possible worlds. Using counterpart theory rather than postulating literal occupation of multiple possible worlds has several advantages. It accommodates apparent failures of transitivity in trans-world identity, but most significantly, counterpart relations sensitive to different aspects of similarity can be used to defuse a range of paradoxes about identity in modal contexts. We are inclined to say a statue cannot survive being flattened, but the piece of clay that makes it up can survive. If we do not want to postulate two objects (the clay statue and the statue-shaped piece of clay), we can accommodate this difference by saying one and the same object has two classes of counterparts: the statue counterparts and the clay counterparts. When we invoke the statue counterpart relation, there are no flattened counterparts, since there are no possible objects similar enough in the right statue-respects that exist through flattening. When we invoke the piece-of-clay respect of similarity, many possible pieces of clay are counterparts despite being flattened.
Lewis defended a materialist metaphysics of mind (see the extended essay on the mind/body problem): mental states were those that played the functional roles defined by common sense psychology, so in humans at least, they were brain states. His argument that mental vocabulary should be understood so that by definition beliefs are whatever the states are that play the causal role specified in folk psychology was influential in the rise of materialism about the mind in the later twentieth century, and other work by him, e.g., on the problem of qualia, were also important.
Lewis published on many other metaphysical topics including: the metaphysics of color, the metaphysics of classes (see class, collection, set), events (see event theory), EXISTENCE, the metaphysics of fiction, FREE WILL (see the extended essay), holes, properties, time travel, TRUTHMAKERS, PART/WHOLE, the problem of the many, and the metaphysics of quantum mechanics. Most of Lewis's papers on metaphysics can be found in Lewis, 1986b and Lewis, 1999.
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