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.