Halliday & Matthiessen (1999: 444):
The view that language distorts the picture of reality, and that there is a mismatch between language and thought, is reflected [in] the opposition of “deep” and “surface” as these figured in a prominent approach to text generation. In this model, “deep generation” is concerned with thinking what to say (the thoughts “behind” the words) and “surface generation” with how to say it.