Abstract
David Lewis famously takes mereology ‘to be perfectly understood, unproblematic, and certain". In this chapter, the author proceeds by articulating four theses that Lewis holds about composition. Three of them are familiar; Lewis himself explicitly articulates and relies upon them. The fourth remains implicit, but it is nonetheless important. The four theses include: 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); and composition is unmysterious (it is not problematic to treat it as primitive, and can function in demystifying explanations). The author devotes 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. Lewis uses the label ‘uniqueness’ and explicitly endorses the thesis.