Halliday & Matthiessen (1999: 189):
… the primary distinction within figures is that between conscious processing and other forms of experience: the key participant in a conscious process, the Senser, is restricted to things that are construed as being endowed with consciousness, so we take conscious/non-conscious as the primary distinction. It is also helpful, in the case of English, to make an initial distinction between objects, which are treated as bounded, and substances, which are not. This gives us an initial categorisation in the form ‘conscious / non-conscious: animals / institutions / objects / substances / abstractions’.