Halliday & Matthiessen (1999: 130):
Sensing and saying construe the "Symboliser" along different lines. The interior Symboliser of sensing is construed as a participant engaged in conscious processing; hence it is endowed with consciousness by virtue of serving in a figure of sensing. … The “Symboliser” of a figure of saying often is a conscious speaker. However, since saying is exterior rather than interior symbolic processing, the Symboliser of saying, unlike that of sensing, is not restricted to human consciousness; it may also be any kind of symbol source, a ‘semiotic thing’ such as institutions, documents and instruments of measurement. … We recognise the difference between a Symboliser of sensing and a Symboliser of saying by calling them Senser and Sayer, respectively.