This essay is from my book Language in Mind and Language in Society (1987). In 1993 Noam Comsky characterised this work as the only attempt known to him to try to explain such notions as "community language" and "abstract language", and he has gone on to cite the book in later publications. The essay identifies five accounts of what a language is: a natural kind, a name (Nominalism), an abstract object (Platonism), a social fact (Sociologism); both a natural kind and a social fact (Dualism). The work of Chomsky, Bickerton, Katz, Labov, Saussure, Itkonen - among others - is discussed at length. The essay tries to establish a framework in which both the psychology of language and the sociology of language can be pursued, and the inter-relations between psychological and socal realities properly understood. It does this by trying to clarify the ontology of different levels of linguistic reality and the methods by which they may be appropriately studied.