Halliday & Matthiessen (1999: 615):
A prerequisite for the semiotic construal of experience is generalisation: the move from “proper” to “common” as the basic principle of referring. … this opens the way (i) to constructing hierarchies of classes … and (ii) to naming other kinds of element, process and qualities, which can be construed only in “common” terms. Since these other elements have distinct and complementary functions it becomes possible to combine them into organic structures, as complex elements or as figures.