Halliday & Matthiessen (1999: 2-3):
What is the significance of this switch of metaphor from knowing to meaning? A meaning base differs from a knowledge base in the direction from which it is construed. In modelling the meaning base we are building it 'upwards' from the grammar, instead of working 'downwards' from some interpretation of experience couched in conceptual terms, and seen as independent of language. We contend that the conception of 'knowledge' as something that exists independently of language, and may then be coded or made manifest in language, is illusory. All knowledge is constituted in semiotic systems, with language as the most central; and all such representations of knowledge are constructed from language in the first place.