Abstract

If extended objects were just sums of unextended parts, no satisfactory theory of contact would be possible. So extended objects are not decomposable into sets of simples. The fact that extended things do not possess a single decomposition into a set of smallest, indivisible parts places important constraints upon the analysis of propositions involving mass terms. In particular, it rules out those which construe masses of matter as set-theoretical constructions out of the parts of things. Consequently, any correct "semantics for mass terms" must recognize as individuals in their own right the masses of matter out of which tables, trees, and human bodies are composed. Although ordinary objects are generally regarded as capable of surviving changes in their parts, masses of matter cannot survive such changes. For instance, if the mass of matter now composing a certain statue does not share every part in common with the mass which made up that statue yesterday, then yesterday's mass of matter is numerically distinct from today's. ;These facts lead to the following argument for mereological essentialism: If there were an object that gained or lost a part, then it would be wholly coincident with a distinct thing--namely, the mass of matter then constituting it. Since masses of matter must count as individuals, and not sets of parts of things that are not themselves masses, objects that gain or lose parts would have to be wholly coincident with objects that cannot. But if both mass and statue weigh 100 pounds, and each is a distinct object in its own right, how is it that I can lift "both" without straining my back? One of the two must be more fundamental, more "real" than the other. Since simple coincidence between mass and constituted object is absurd, there are no objects that gain or lose parts. Part-changing objects are either fictions, logical constructions, or, perhaps, "disturbances" occurring to masses of matter. Neither relative identity theories, temporary identity theories, nor the doctrine of temporal parts can provide the resources to block this argument.