Halliday & Matthiessen (1999: 521):
… in a projecting relationship, the two elements in a verbal projection are typically equal in status, while those in a mental projection are typically unequal. This is not surprising: since you can hear what a person says, you give the wording the full status of a direct experience … whereas you cannot observe what a person thinks, so this is more likely to be construed as dependent on the process that projects it …