Halliday & Matthiessen (1999: 521):
In projection, one process is used to construe another one, such that the latter becomes a representation of what someone says or thinks. The types of process that have this power of projection are the verbal and mental processes … Thus the projection operates at either of the two strata of the content plane: either that of the wording, where the projection is by a verbal process, or at that of the meaning, where it is by a mental process. Because the grammar can project in this way, semiotic events, both those which are externalised as sayings and those which are internalised as thoughts, are brought within the overall domain of the phenomena of experience.