Abstract

The thesis that ordinary material objects are mereological sums in the standard sense has been remarkably popular among three-dimensionalists and four-dimensionalists alike. This chapter considers two prominent representatives: Judith Jarvis Thomson, for the three-dimensionalist camp; and David Lewis, for the four-dimensionalist camp. The question of why Thomson's temporalized and modalized version of standard mereology still does not adequately capture the characteristics of ordinary material objects is postponed until Chapter 4. The remainder of the chapter considers Lewis' argument in favor of Unrestricted Composition, in particular in the recently expanded version offered in Theodore Sider's, _Four-Dimensionalism: An Ontology of Persistence and Time_; as well as Lewis' defence of the Composition-as-Identity Thesis in his _Parts of Classes_. It is argued that the three-dimensionalist need not be swayed by either component of Lewis' view, since they are ultimately founded on question-begging reasoning.