Halliday & Matthiessen (2014: 214):
There is a basic difference, that we become aware of at a very early age (three to four months), between inner and outer experience: between what we experience as going on ‘out there’, in the world around us, and what we experience as going on inside ourselves, in the world of consciousness (including perception, emotion and imagination). The prototypical form of the ‘outer’ experience is that of actions and events: things happen, and people or other actors do things, or make them happen. The ‘inner’ experience is harder to sort out; but it is partly a kind of replay of the outer, recording it, reacting to it, reflecting on it, and partly a separate awareness of our states of being. The grammar sets up a discontinuity between these two: it distinguishes rather clearly between the outer experience, the processes of the external world, and inner experience, the processes of consciousness. The grammatical categories are those of material process clauses and mental process clauses …