Halliday & Matthiessen (1999: 494):
… the Medium is restricted relative to the Process in the same way regardless of whether the clause is effective (+ Agent) or middle …the Medium is the participant that is most bonded to the Process by lexical collocation.the Medium is the element which combines with the Process to form the clause nucleus; it is the nucleus which determines how the process is subclassified and interpreted …the Medium is the one participant that cannot be treated circumstantially; hence (other than one special kind of clause, the medio-passive) it is never mediated by a preposition.