Halliday (2008: 80):
So the analogy whereby language is to text as climate is to weather is useful to think with — up to a point. It reminds us that there are not two different things at issue here, but only one. What we call “climate”, and what we call “weather”, are the same phenomenon seen from different angles, or different moments of time. So it is with “language” and “text”. But in the case of language, so much misunderstanding has been caused by counterposing these two terms, with language and text being treated as if they were different orders of reality, that I shall not make use of these terms in opposition. Instead I shall talk of “text” and “system”, these being two aspects, or guises, of the single phenomenon “language”: thus, language as system, and language as text. Here I am borrowing “system” from its place in the generalised opposition between system and instance.