Entities of many kinds, not just material things, have been credited with parts. Armstrong, for example, has taken propositions and properties to be parts of their conjunctions, sets to be parts of sets that include them, and geographical regions and events to be parts of regions and events that contain them. The justification for bringing all these diverse relations under a single ‘part–whole’ concept is that they share all or most of the formal features articulated in mereology. But the concept has also prompted an ontological thesis that has been expressed in various ways: that wholes are ‘no ontological addition’ to their parts ; that to list both a whole and its parts is ‘double counting’; and that there is ‘no more’ to a whole than its parts: for example, that there is no more to a conjunction than the conjuncts that are its parts, and whose truth or falsity determines whether it is true or false. For brevity, I shall express the thesis in the last of these ways, as the claim that entities with parts are ‘nothing but’ those parts.