Halliday & Matthiessen (1999: 110-1):
Let us look more closely at these two modes of projecting, in both of which the projecting figure represents symbolic processing, the processing that brings the other figure into symbolic existence. Either the projection takes the prototypical form of semiosis: it is presented as verbal, shared, an exchange or joint construction of meanings; or it is fashioned into a derived semiotic form, unshared, interiorised, and without any meaning being exchanged. In the first, the projecting figure is one of saying; the projected is referred to as locution. In the second, the projecting figure is one of sensing; the projected is referred to as idea. Ideas are projections which are sensed, locutions are projections which are said.