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. Thus in an example such as the thermometer thinks it is 35 degrees, the Symboliser has to be interpreted as if it was a conscious being.
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 (see Halliday, 1985:129-30). Thus alongside examples such as
In the hospital's newsletter, he tells of one patient who stopped a two-week-long bout.
we also find
The British medical Journal The Lancet recently reported a study at Oxford university's John Radcliffe Hospital.
And while the thermometer thinks it is 35 degrees requires a metaphorical reading, the thermometer says it is 35 degrees does not. We recognise the difference between a Symboliser of sensing and a Symboliser of saying by calling them Senser and Sayer, respectively.