In any piece of discourse analysis, there are always two possible levels of achievement to aim at. One is a contribution to the 'understanding' of the text: the linguistic analysis enables one to show how, and why, the text means what it does. In the process, there are likely to be revealed multiple meanings, alternatives, ambiguities, metaphors and so on. This is the lower of the two levels; it is one that should always attainable provided the analysis is such as to relate the text to general features of the language – provided it is based on a grammar, in other words. The higher level of achievement is a contribution to the 'evaluation' of the text: the linguistic analysis may enable one to say why the text is, or is not, an effective text for its own purposes – in what respect it succeeds and in what respects it fails, or is less succesful. This goal is very much harder to attain. It assumes an interpretation not only of the environment of the text, its ʻcontext of situationʼ and ʻcontext of cultureʼ, but also of how the linguistic features of a text relate systematically to the features of its environment, including the intentions of those involved in its production.
- Halliday (1985), a pag.XV-XVI