First, Chomsky has fundamentally restructured grammatical research. Due to his work, the central object of study in linguistics is ‘the language faculty’, a postulated mental organ which is dedicated to acquiring linguistic knowledge and is involved in various aspects of language-use, including the production and understanding of utterances. The aim of linguistic theory is to describe the initial state of this faculty and how it changes with exposure to linguistic data. Chomsky (1981) characterizes the initial state of the language faculty as a set of principles and parameters. Language acquisition consists in setting these open parameter values on the basis of linguistic data available to a child. The initial state of the system is a Universal Grammar (UG): a super-recipe for concocting language-specific grammars. Grammars constitute the knowledge of particular languages that result when parametric values are fixed.
Linguistic theory, given these views, has a double mission. First, it aims to ‘adequately’ characterize the grammars (and hence the mental states) attained by native speakers. Theories are ‘descriptively adequate’ if they attain this goal. In addition, linguistic theory aims to explain how grammatical competence is attained. Theories are ‘explanatorily adequate’ if they show how descriptively adequate grammars can arise on the basis of exposure to ‘primary linguistic data’ (PLD): the data children are exposed to and use in attaining their native grammars. Explanatory adequacy rests on an articulated theory of UG, and in particular a detailed theory of the general principles and open parameters that characterize the initial state of the language faculty (that is, the biologically endowed mental structures).