Halliday & Matthiessen (1999: 503-4):
In our treatment of figures in the ideation base, we stressed the fluidity of the boundaries among the various types of figure: doing-&-happening, sensing, saying, being-&-having. We construe these in the grammar as a system of process types: at primary delicacy, material, mental, verbal, relational. These are sections on a continuum — or, better, regions in an n-dimensional semantic space; but they are not demarcated by any uniquely self-selecting set of criteria.
A stratified semiotic defines three perspectives, which (following the most familiar metaphor) we refer to as 'from above', 'from roundabout', and 'from below':
looking at a given stratum from above means treating it as the expression of some content,looking at it from below means treating it as the content of some expression, whilelooking at it from roundabout means treating it in the context of (i.e. in relation to other features of) its own stratum.
Adopting this trinocular perspective we have identified the various process types in terms of their nuclear transitivity; this seems to us the most operationally useful approach, since it takes account of
what they mean,how they are expressed, andwhat their systemic potential is
— without privileging any one perspective at the expense of the other two.