Halliday & Matthiessen (1999: 278):
We noted above that the deployment of spatial metaphor in the construction of economic ‘knowledge’ serves, among other things, as an interface to another form of semiotic — that of using space symbolically in a diagrammatic representation of quantities. The same kind of principle of cross-over between semiotic modes applies to the construal of meaning in linguistics. We have relied in our own discussion on metaphors of abstract space for construing meaning — most centrally, metaphors of semantic networks and semantic space. These metaphors allow us to cross over to diagrams of symbolic space, viz. system networks (as a kind of acyclic directed graph) and topological representations. The system networks can, in turn, be restated algebraically, as in the various computational implementations of systemic accounts.