What the child does with language tends to determine its structure. This relatively close match between structure and function can be brought out by a functional analysis of the system, in terms of its meaning potential. We can see from this how the structures that the child has mastered are direct reflections of the functions that language serves for him.
- Halliday (1973), a pag.27

The analysis that we have offered is a functional one in the two distinct but related senses in which the term ʻfunctionalʼ is used in linguistics. It is an account of the functions of language; and at the same time the structures are expressed in terms of functional elements (and not of classes, such as noun and verb). It could be thought of as a kind of ʻcase grammarʼ, although the structural parts are strictly speaking ʻelements of structureʼ (as in system-structure theory) rather than ʻcasesʼ; they are specific to the context (i.e. to the particular function of language, in this instance), and they account for the entire structure, whereas cases are contextually undifferetiated and also restricted to elements that are syntactycally dependent on a verb.
- Halliday (1973), a pag.29