The theme is what is being talked about, the point of departure for the clause as message, and the speaker has within certain limits the option of se… - Michael Halliday

A child learning his [her] mother tongue is learning how to name; he [she] is building up a meaning potential in respect of a limited number of social functions. These functions [instrumental, regulatory, interactional, personal, heuristic, and imaginative] constitute the semiotic environment of a very small child, and may be thought of as universals of human culture

p. xiv cited in: Piet Van de Craen (2007) Van Brussel gesproken. p. 118.

It is part of the task of linguistics to describe texts, and all texts, including those prose or verse, which fall within any definition of literature and are accessible to analysis by the existing methods of linguistics.