 The theory obtained by adding (P.5) to (P.1)–(P.3) is thus a proper extension of MM. We label this stronger theory EM, for Extensional Mereology, the attribute ‘extensional’ being justified precisely by the exclusion of countermodels that, like the one in Figure 4, contain distinct objects with the same proper parts. In fact, it is a theorem of EM that no composite objects with the same proper parts can be distinguished: