Halliday (2008: 80):
In the same — or at least an analogous — sense [as climate being a theory of the weather], language is a theory of the text. Again, it may be a low-level commonsense theory, as when we comment on a particular person’s language, or on the language of advertisers or of politicians. … Or it may be a designed scientific theory, such as we find in learned articles on the grammar or phonology of a given language, or of several languages or even language in general. When this happens we turn the theory itself into a virtual entity and call it “linguistics”, or, in the case of the weather, “climatology”.