In spoken language, we perform without thinking. Talking is like walking (and developmentally the two go together; protolanguage goes with crawling, language with walking): if you think about it, you stumble (which is a metaphor we often use). This means that the categories of our language represent unconscious rather than conscious slices of meaning; and this is one of the main problems for a grammatical theory [...] the category only exists in the unconscious semantic system of the English language [...] The meaning is built into our unconscious. This does not mean it cannot be learnt; but it can only be learnt in use.
- Halliday (1985), Pag. XXV-XXVI