Abstract

Structural properties are properties something has in virtue of its mereological structure in that they are properties whose instantiation by a particular involves the parts of the particular being propertied and related in the appropriate way. Most of the literature on structural properties has focused on problems that arise from the pairing of two assumptions: (1) structural properties are universals and (2) structural properties are, in some sense, composed of the properties they involve. Chief among these difficulties is David Lewis’ claim that the conjunction of (1) and (2) require non-unique composition and hence entail the denial of the extensionality of parthood: butane, as well as methane, is composed of the universals carbon, hydrogen and bonded, yet they are distinct properties. This is the extensionality problem. In this paper I discuss a different problem for the conjunction of (1) and (2) and propose a single solution to both problems. First, in section two, I introduce the idea of multiple decomposition and suggest that at least some structural properties are multiply decomposable. In section three, I introduce the “problem of multiple decompositions.” This problem arises when it seems possible for a structural property to be composed of some xs and some ys even when the xs ≠ ys. Finally, in section four I show how both problems admit of a single solution by focusing on the parthood relation that holds between structural properties and their constituents. While the solution I propose is theoretically costly, I argue that this cost should be paid to retain both (1) and (2).