Halliday & Matthiessen (1999: 606):
These systems enter into relation with language in two ways. On the one hand, they are metonymic to language: they are complementary, non-linguistic resources whereby higher–level systems may be realised … On the other hand, they relate metaphorically to language: they are constructed, stratally and metafunctionally, in the image of language itself, and hence can be modelled on language as prototype, being described “as if” they had their own grammar and semantics …