Halliday & Matthiessen (1999: xi):
Condensed into one short paragraph, our own point of departure is the following. Language evolved, in the human species, in two complementary functions: construing experience, and enacting social processes. In this book we are concerned with the first of these, which we refer to as constructing the "ideation base"; and we stress that the categories and relations of experience are not "given" to us by nature, to be passively reflected in our language, but are actively constructed by language, with the lexicogrammar as the driving force. By virtue of its unique properties as a stratified semiotic system, language is able to transform experience into meaning. In our attempt to describe this process, we have deliberately used the grammar as the source of modelling, because we wanted to show how such a process could take place. We have confined ourselves, in principle, to how it takes place in English; the theoretical concepts we have used are general to all languages, but the descriptive categories should be interpreted in the context of a description of English.