Matter: Definition, Realisation & WH– Probe

Halliday & Matthiessen (2014: 327-8):

Matter is related to verbal processes; it is the circumstantial equivalent of the Verbiage, ‘that which is described, referred to, narrated, etc’.  The interrogative is what about?.  Matter is expressed by [prepositional phrases introduced by] prepositions such as about, concerning, with reference to and sometimes simply of … It is frequent with both ‘verbal’ clauses and ‘mental’ ones (especially of the ‘cognitive’ subtype).