Halliday & Matthiessen (1999: 26):
Thus when we move from the lexicogrammar into the semantics, as we are doing here, we are not simply relabelling everything in a new terminological guise. We shall stress the fundamental relationship between (say) clause complex in the grammar and sequence in the semantics, precisely because the two originate as one: a theory of logical relationships between processes. But, as we have shown, what makes such a theory (i.e. an ideation base as the construal of experience) possible is that it is a stratal construction that can also be deconstructed, every such occasion being a gateway to the creation of further meanings which reconstrue in new and divergent ways. Thus a sequence is not 'the same thing as' a clause complex; if it was, language would not be a dynamic open system of the kind that it is. This issue will be foregrounded particularly in our discussion of grammatical metaphor.