 Berto &amp; Plebani (2015) provide an useful introduction to ontology and metaontology. Putnam (1971) makes the case for abstract objects on scientific grounds. Bealer (1993) and Tennant (1997) present a priori arguments for the necessary existence of abstract entities. Fine (2002) systematically studies of abstraction principles in the foundations of mathematics. Wetzel (2009) examines the type-token distinction, argues that types are abstract objects while the tokens of those types are their concrete instances, and shows how difficult it is to paraphrase away the many references to types that occur in the sciences and natural language. Zalta (2020) develops a type-theoretic framework for higher-order abstract objects (which includes abstract properties and abstract relations in addition to ordinary properties and relations) and offers both comparisons to other type theories and applications in philosophy and linguistics. Moltmann (2013) investigates the extent to which abstract objects are needed when developing a semantics of natural language; in this book, and also in her article (2020), she defends a ‘core-periphery’ distinction and suggests that natural language ontology contains references to abstract objects only in its periphery. Falguera and Martínez-Vidal (2020) have edited a volume in which contributors present positions and debates about abstract objects of different kinds and categories, in different fields in philosophy.