[interpersonal meaning] embodies all use of language to express social and personal relations, including all forms of the speaker's intrusion into th… - Michael Halliday

What makes learning possible is that the coding imposed by the mother tongue corresponds to a possible mode of perception and interpretation of the environment. A green car can be analysed experientially as carness qualified by greenness, if that is the way the system works.

We should not, perhaps, take it for granted that a description in terms of a formalised model, which has certain properties lacking in those derived from models of another kind, will necessarily be the best description for all of the very diverse purposes for which the descriptions of languages are needed. In assessing the value of a description, it is reasonable to ask whether it has proved useful for the purposes for which it was intended.

[The construal of context as a semiotic construct accompanies the construal of language as a metafunctionally organized system because of the realizational relationship between the two. And it is this relationship which helps to explain how language is learned:]
It is this that enables, and disposes, the child to learn the lexicogrammar: since the system is organized along functional lines, it relates closely to what the child can see language doing as he observes it going on around him.