Human language, that unique characteristic of our species, has been of interest throughout history. The scientific study of human language is called linguistics. A linguist, then, is not someone who speaks many languages ( althought many linguist do) ; such individuals are polyglots. A linguist is a scientist who investigates human language in all its facets, its structure, its use, its history, its place in society.
The form and structure of the kinds of linguistic knowledge speakers possess is the concern of the theoretical lingusitics. This theory of grammar - the mental representation of linguistic knowledge - is what this textbook is about. But the field of linguistics is not limited to grammatical theory ; it includes a large number of subfields, which is true of most sciences concered with phenomena as complex as human language.