Halliday & Matthiessen (1999: 508):
Physical systems are just physical systems. Biological systems, however, are not just biological systems; they are at once both biological and physical. Social systems are all three: social, biological and social. This makes them increasingly difficult for us to comprehend. This is not the same as saying that social systems are more complex than biological ones, or biological than physical; there are too many ways in which things can be complex, for any such observation to make sense. But they are increasingly complex in this particular respect; and this means that it is increasingly difficult to recognise the essential nature of the phenomena concerned.