Halliday & Matthiessen (1999: 509):
It can be studied as a physical system, in acoustics and in the physical aspect of articulation (air pressure measurements and so on). It can be studied as a biological system, in the physiological aspects of articulation and in the neurophysiology of the brain. It can be studied as a social system, as the primary mode of human interaction. And of course it can be studied as a semiotic system, in the core areas of lexicogrammar, phonology and semantics.