Halliday & Matthiessen (1999: 429):
It follows then … that for us [Fawcett’s (extra-linguistic)] “knowledge of the universe” is construed as meaning rather than as knowledge. This meaning is in the first instance created in language; but we have noted that meaning is created in other semiotic systems as well, both other social-semiotic systems and other semiotic systems such as perception. Our account gives language more of a central integrative rôle in the overall system. It is the one semiotic system which is able to construe meanings from semiotic systems in general.
Halliday & Matthiessen (1999: 3):
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.