Halliday & Matthiessen (2004: 23):
Systemic theory gets its name from the fact that the grammar of a language is represented in the form of system networks, not as an inventory of structures. …
structure … is interpreted as the outward form taken by systemic choices, not as the defining characteristic of language.
1. Because meaning is intrinsically paradigmatic:
Halliday & Matthiessen (1999: 509):
Meaning can only be construed symbolically, because it is intrinsically paradigmatic, as Saussure understood and built into his own definition of valeur.
2. Because explaining functionality gives priority to the view 'from above':
Halliday & Matthiessen (2004: 31):
Giving priority to the view ‘from above’ means that the organising principle adopted is one of system: the grammar is seen as a network of interrelated meaningful choices. In other words, the dominant axis is the paradigmatic one: the fundamental components of the grammar are sets of mutually defining contrastive features. Explaining something consists not of stating how it is structured but in showing how it is related to other things: its pattern of systemic relationships, or agnateness (agnation).
3. Because evolved systems cannot be explained simply as the sum of their parts:
Halliday & Matthiessen (2004: 20):
… languages evolve — they are not designed, and evolved systems cannot be explained simply as the sum of their parts. Our traditional compositional thinking about language needs to be, if not replaced by, at least complemented by a ‘systems’ thinking whereby we seek to understand the nature and the dynamic of a semiotic system as a whole.