 Quine (1957 [1969: 23]) famously claimed that there should be no entity without identity. His paradigmatic case concerns sets: two of them are identical iff they have exactly the same members. Since then it has been customary in ontology to search for identity conditions for given categories of entities and to rule out categories for want of identity conditions (against this, see Lowe 1989). Quine started this trend precisely by arguing against properties and this has strictly intertwined the issues of which properties there are and of their identity  conditions.[13]