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 literatur… - Michael Halliday

Language... is a range of possibilities, and an open end set of options in behavior that are available to the individual in his existence as social man. The context of culture is the environment of any particular selection that is made within them... The context of culture defines the potential, the range of possibilities that are open. The actual choice among these possibilities takes place within a given context of situation.

Any text in spoken English is organized into what may be called 'information units'. (...) this is not determined (...) by constituent structure. Rather could it be said that the distribution of information specifies a distinct structure on a different plan. (...) Information structure is realized phonologically by 'tonality', the distribution of the text into tone groups.

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 selecting any element in the clause as thematic.