Functionalist approaches to linguistics rest on the fundamental assumption, underlying a broad spectrum of work, that language is shaped by its use. Functionalism represents a point of departure rather than a unified theory or codified model of language, but it does have important theoretical implications. It implies that the ultimate goal of linguistics goes beyond description (as in structuralism) and even generalization (as in typology) to explanation of an inclusive kind. Of course much modern linguistic theory seeks to be explanatory in some sense. Under some approaches, explanation has been framed chiefly in terms of theory-internal consistency. A model of language is constructed and described in terms of abstract, inherent structural principles. Individual constructions are then explained by their conformity with the principles. Functional explanations have tended to be wider ranging, encompassing both language-internal and language-external considerations. Linguistic structures are seen to be shaped by a variety of forces, including the many physiological, cognitive, and contextual factors involved in their acquisition and use. Pertinent physiological factors include, for example, the motor abilities that constrain articulation.