Halliday & Matthiessen (1999: 513-4):
At the same time, while recognising a general category of “process” to construe our experience of change, the grammar also recognises that not all processes are alike. As human beings we become aware (and again we can see this in the actions of tiny infants) that phenomena fall into two distinct types: those happening outside ourselves, which we can see and hear, and those happening within our own consciousness — thoughts and feelings, and also the sensations of seeing and hearing, as distinct from whatever is seen or heard. The grammar construes this as a distinction between “material processes” and “mental processes”. Mental processes are specifically attributed to conscious beings …