Halliday & Matthiessen (1999: 507):
There is an ordering among these four types of system, which we can appreciate most readily, perhaps, at the “meta” level: this is the order in which they have come to be studied and interpreted, in the past five hundred years of human scholarship. […] In other words, there was a certain intellectual distance to be covered in bringing a comparable kind of insight into these different types of system–&–process.