Halliday & Matthiessen (1999: 135):
Mental and verbal clauses are distinct from material and relational clauses in that the former can project ideas and locutions (quote or report). These represent the ‘content’ of sensing and saying, as in David thought —> the moon was a balloon, where the relation of projection is represented by an arrow. Verbal clauses are distinct from mental clauses in that the Sayer is not necessarily an entity endowed with consciousness; and in verbal clauses there may be a further participant, the Receiver, which is not found in a mental clause.** Except, of course, where sensing is construed as interior saying: he thought to himself.