, and Certain ” : Lewis on Mereology

David Lewis famously takes mereology " to be perfectly understood, unproblematic, and certain " (1991, 75). It is central to his thought, appearing in his discussions of set theory, modality, vagueness, structural universals, and elsewhere. He held views not only about how composition works and when it occurs, but also about the role of mereology in philosophy. In this essay, I will proceed by articulating four theses that Lewis holds about composition. (I would call them the four U's, if only 'unguilty' were a word!) Three of them are familiar; Lewis himself explicitly articulates and relies upon them. The fourth remains implicit, but it is nonetheless important. Here they are: Composition is unique—the same things cannot have two different fusions. Composition is unrestricted—any two things whatsoever have a fusion. Composition is ontologically innocent—composed entities do not " count " beyond their parts. Composition is unmysterious—it is not problematic to treat it as primitive, and can function in demystifying explanations. I will devote a section to each thesis: explaining what it says, pointing to the texts that illustrate that Lewis believes it, and explaining why Lewis believes it. These sections are largely expository. But woven in between them are interstitial sections in which I reflect upon further questions that arise, and draw further lessons. Two preliminary caveats. First, I will often talk of 'things' or 'objects' either having parts or being parts of something else. This is not meant to suggest that only concrete physical objects stand in the parthood relation. Lewis allows that things that do not occupy space and time can also do so (1991, 75), and I will follow him in this. My choice of words is just that—a choice of words, intended to be neutral about just what kinds of things can be part of others. Second, the four theses around which this piece is structured do not exhaust Lewis' claims about mereology. He also believes, for example, that parthood is transitive (e.g. 1991, 74) and that atomless gunk—entities all of whose parts have proper parts—is possible (e.g. 1986a, 30; 1991, 20-21). I have simply chosen the four theses because they are particularly interesting and revealing.