This book is intended as a contribution to linguistic theory in the broadest sense. It offers a view of language - illustrated through an examination of the linguistics of number - which brings together considerations of individual psychology and of communication within a speech community. These two strands, the psychological and the social, are put together to give an evolutionary perspective on language. The psychological considerations relate both to the invention and to the ordinary acquisition of language; the social considerations relate to the ways individuals negotiate common standard expressions for their meanings. Languages, the author argues, grow through the interaction of individual minds on the forms invented and socially negotiated by their predecessors. The book also makes a contribution to the philosophy of numbers; arguing that our knowledge of numbers is akin to our possession of language, and that both emerge from a faculty for constructing collections from aggregates.