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 [impinging] (or is brought into (mental) existence by the Senser’s conscious processing [emanating]). Alternatively, the Senser’s conscious processing may project another figure [idea] within the same sequence.