The combination of system and structure with rank leads to a fairly abstract grammar (fairly ʻdeepʼ, in the Chomskyan sense) and enables us to specify fairly accurately in theoretical terms – though not of course in rule-of-thumb terms–--just how abstract it is. In principle, a grammatical system is as abstract (is as ʻsemanticʼ) as possible given only that it can generate integrated structures; that is, that its output can be expressed in terms of functions which can be mapped directly on to other functions, the result being a single structural ʻshapeʼ (though one which is of course multiply labelled). This is already fairly abstract, and it may be unnecessary therefore to interpose another layer of structure between the semantic systems and the grammatical systems–---given the limited purpose of the semantic system, which is to account for the meaning potential associated with defined social contexts and settings.
- Halliday (1973), a pag.95