Halliday & Matthiessen (1999: 503):
… we treat transitivity both within semantics (the paradigmatic and syntagmatic organisation of figures) and within lexicogrammar (the grammar of transitivity): it is a system construed within the content plane of language — both in the ideational component in the lexicogrammar and in the ideation base. This two-stratal approach to transitivity makes it possible to model the resource of grammatical metaphor and is fundamental to work on multilingual systems for generating text.