Halliday & Matthiessen (1999: 151):
Here there is one participant, the Senser, who is construed as a conscious being engaged in "inert" conscious processing ("sensing", as distinct from conscious processing as a form of active behaviour). This may involve another participant, the Phenomenon, which enters into the consciousness of the Senser (or is brought into (mental) existence by the Senser's conscious processing). Alternatively, the Senser's conscious processing may project another figure [idea] within the same sequence.