Halliday & Matthiessen (1999: 11):
… the present book is concerned with just one portion of the higher-level environment of the grammar, that having to do with the ideational metafunction. In other words, we are concerned with that portion of the semantics which "controls" the ideational systems in the grammar, primarily, that of transitivity in the clause and those of projection and expansion in the clause complex. Transitivity is the grammar of processes: actions and events, mental processes and relations. It is that part of grammar which constitutes a theory of "goings-on". Projection and expansion are the fundamental relations between processes: this is the part of the grammar that constitutes a theory of how one happening may be related to another. Thus our aim is towards a general ideational semantics. We may call this the ideational meaning base, or ideation base for short. It is complemented by meaning bases supporting the other two metafunctions — the interaction base (supporting the interpersonal metafunction) and the text base (supporting the textual metafunction).
Blogger Comments:
The term 'controls' is in scare quotes because the theoretical relation between semantics and grammar is not enhancement (cause), but elaboration (intensive identification).