'Shared knowledge' and topicality

The 'shared knowledge' often associated with specific linguistic forms, such as definite descriptions, cleft constructions and specific intonation contours is shown to be a function of the role of these constructions in encoding the topic-comment structure of a sentence. It is argued that this explains certain properties of the relation between shared knowledge and linguistic form, in particular: (1) why 'shared knowledge' is associated with some forms and not with others; (2) why 'shared knowledge' is not consistently associated even with these forms; and (3) why what is assumed to be 'shared' is familiarity with a discourse entity and not necessarily belief in the truth of a corresponding proposition.