The Large-Scale Joints of the World
What is the compositional structure of reality? That question divides naturally into these two: What is the compositional structure of the particulars that populate reality? And what is the structure of the properties and relations that fix what these entities are like? David Lewis‘s work in ontology and mereology provides the materials for an extraordinarily clean answer to the first question. First, among the particulars1 that populate reality are mereological simples: entities that have no proper parts. (A plausible candidate for these simples: spacetime points.) Second, every collection of such entities has a unique mereological fusion. And third, every particular is either a simple, or a fusion of simples.2 That‘s it. I propose to take this answer on board.3 What, then, about our second question? Here it looks as though we can draw on an additional Lewisian thesis:
Joints: There is a distinction — at the level of metaphysics — between more and less natural properties. Some properties (having mass 1 gram, perhaps) are perfectly natural; others (being a methane molecule, perhaps) are less-but-still-quite natural; still others (being grue is a favorite) are not very natural at all. This distinction earns its philosophical keep because of the number and centrality of the philosophical projects that must presuppose it. And to say that this distinction resides at the level of metaphysics is, at least in part, to say that it is not grounded in facts about human psychology.
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