1 Material Constitution Is Ad Hoc Jeroen Smid Lund University [Final version appears in Erkenntnis DOI: 10.1007/s10670-016-9818-6] Abstract The idea that two objects can coincide—by sharing all their proper parts, or matter—yet be non-identical, results in the “Problem of Coincident Objects”: in what relation do objects stand if they are not identical but share all their proper parts? One solution is to introduce material constitution. In this paper, I argue that this is ad hoc since, first, this solution cannot be generalized to solve similar (real) problems, and, second, there are pseudo cases of coincidence that should not trigger the introduction of material constitution and these cannot be distinguished in any principled way from the allegedly real cases of coincidence. Keywords: Material constitution, coincidence, ad hoc, theory-choice, mereology. 0. Introduction Material constitution is introduced to make sense of the idea that two objects can coincide (for example, by sharing all their proper parts) yet be non-identical. The claim that such cases of coincidence are genuine is often advocated on the basis of a Leibniz’s Law-style argument showing that, for example, a statue has certain aesthetic and modal properties that the statue’s clay lacks. Constitution then explains how it can be that two objects are non-identical yet share the same location and many properties (such as mass and shape). In this paper I argue that constitution provides an ad hoc solution here. First, I show that while it solves one particular problem, constitution cannot solve certain relevantly similar problems. Other theories, however, can readily deal with this difficulty. Hence, second, I explain that constitution should be applicable only in genuine cases of coincidence, and I then argue that there is no principled way to distinguish allegedly genuine cases of coincidence from pseudo cases of coincidence where one object is referred to by different names. Without such a principled difference there is no reason to think that there are real cases of coincidence, and therefore there is no need to introduce constitution.