The paper argues that very different part-whole relations hold between different kinds of entities. While these relations share most of their formal properties, they need not share all of them. Nor need other mereological principles be true of all kinds of part–whole pairs. In particular, it is argued that the principle of unrestricted composition, that any two or more entities have a mereological sum, while true of sets and propositions, is false of things and events.

Absolute identity seems at first sight to be presupposed in the branch of formal logic called identity theory. Classical identity theory may be obtained by adjoining a single schema to ordinary quantification theory.