Halliday (2008: 192):
The system and the text are not two different phenomena: what we call the “system” of a language is equivalent to its “text potential”. Analysing discourse means, first and foremost, relating the text to the potential that lies behind it. There are perhaps three areas of discourse analysis that have figured prominently in systemic-functional research: literary-æsthetic, technical-scientific, and sociopolitical. In the first of these, the text carries value in its own right; when you analyse texts of this kind, you are aiming to explain not only why and how the text means what it does but also why it carries the value that it does. […] In analysing scientific and technical texts, the linguist is likely to be foregrounding the special properties that distinguish these texts from other varieties of written and spoken language, such that they are able to play a central part in the creation and transmission of knowledge. Scientific theories evolve from the conjunction of material and semiotic processes, and the advancement of science is powered by linguistic as well as technical resources. […] On the socio-political side, the researcher is investigating how discourse creates, maintains and transmits the social order (and hence may also be used to subvert it).