Halliday & Matthiessen (2014: 29):
If we now come back to the question of stratification, we can perhaps see more clearly what it means to say that the semantic stratum is language interfacing with the nonlinguistic (prototypically material) world. Most texts in adult life do not relate directly to the objects and events in their environment. … Interfacing with the eco-social environment is a property of language as system; it is also, crucially, a feature of those instances through which small children come to master the system; but it is not something that is re-enacted in every text. Experience is remembered, imagined, abstracted, metaphorised and mythologised — the text has the power to create its own environment; but it has this power because of the way the system has evolved, by making meaning out of the environment as it was given.