Linguistic theory is concerned primarily with an ideal speaker-listener， in a completely homogeneous speech-community， who knows its language perfectly and is unaffected by such grammatically irrelevant conditions as memory limitations， distractions， shifts of attention and interest，and errors(random or characteristic) in applying his knowledge of the language in actual performance. This seems to me to have been the position of the founders of modern general linguistics， and no cogent reason for modifying it has been offered. To study actual linguistic performance， we must consider the interaction of a variety of factors， of which the underlying competence of the speaker-hearer is only one. In this respect， study of language is no different from empirical investigation of other complex phenomena.
　Aspects of the Theory of Syntax Noam Chomsky