Halliday & Matthiessen (1999: 106-7):
… typically only humans can project into second-order reality. However, since human consciousness is the locus of semiotic activity, it has the power of interpreting as metaphenomenon that which is manifested by some other, non-conscious symbolic source. Thus while “sensing” (that is semiotic activity that is unmanifested, like thinking) does require a human senser, saying can be ascribed to a non-human as well as to a human sayer …