Blogger Comments:
To be clear, the motivation for stratifying the entire content plane into semantic and lexicogrammatical systems is grammatical metaphor. So in the case of Halliday's semantic system of SPEECH FUNCTION and grammatical system of MOOD, interpersonal metaphors of mood motivate their stratification. Halliday & Matthiessen (1999: 237):
If the congruent form had been the only form of construal, we would probably not have needed to think of semantics and grammar as two separate strata: they would be merely two facets of the content plane, interpreted on the one hand as function and on the other as form.