One of the central theoretical concerns of present-day linguistics is the question of whether grammatical knowledge represents an autonomous component of the human mind and is therefore independent of interactional (or any other non-grammatical) competence.

This view is endorsed by the Chomskian school in linguistics which claims that the structures of the languages of the world are the result of selections ("parameter settings") within a genetically determined bio-programme which has developed together with homo sapiens without being functionally motivated. Grammar, according to this view, is an instrument for representation, reasoning and (perhaps accidentally) for communication, but its structure cannot be explained by the way it is used. As a consequence, there are no homologies between grammar, (non-linguistic) cognition and interactional structure.

Against this view, I will argue in this paper that there are fundamental common features shared by interaction and grammar which suggest some kind of interdependence between the two, and thereby a functional explanation of the origin of language; one of these fundamental common features is that of projectability. Human interaction rests on the possibility of projection; the grammars of human languages provide interlocutors with sedimentated and shared ways of organising them.