By a functional theory of language I mean one which attempts to explain linguistic structure, and linguistic phenomena, by reference to the notion that language plays a certain part in our lives; that it is required to serve certain universal types of demand. I find this approach valuable in general for the insight it gives into the nature and use of language, but particularly so in the context of stylistic studies.
- Halliday (1973), Pag. 104

A functional theory of language is a theory about meanings, not about words or constructions; we shall not attempt to assign a word or a construction directly to one function ot another. Where then do we find the functions differentiated in language? They are differentiated semantically, as different areas of what I called the ʻmeaning potentialʼ.
- Halliday (1973), Pag. 110

The essential feature of a functional theory is not that it enables us to enumerate and classify the functions of speech acts, but that it provides a basis for explaining the nature of the language system, since the system itself reflects the functions that it has evolved to serve. The organization of options in the grammar of natural languages seems to rest very clearly on a functional basis, as has emerged from the work of those linguists, particularly of the Prague school, who have been aware that the notion ʻfunctions of languageʼ is not to be equated merely with a theory of language use but expresses the principle behind the organization of the linguistic system.
- Halliday (1973), Pag. 65-66