Halliday & Matthiessen (1999: 508):
What is problematic is the relationship between the system and the instance; or, to put it another way, what is the nature of a “fact” in these different realms of experience? A biological fact is different from a physical fact, and a social fact is different again; the relationship between that which can be observed, and the system–&–process lying behind what is observed, is significantly harder to establish when the system is a social system, because the phenomena involved are simultaneously all three kinds.