Halliday & Matthiessen (1999: 615):
This principle of generalisation — that is, naming general classes rather than specific individuals — is what makes it possible to construct an ideation base. When they have reached this stage, children can make the transition from protolanguage to mother tongue, building up figures and sequences of figures, and simultaneously structuring these as moves in dialogic exchanges (… the interaction base) and as messages or quanta of information (the text base). In other words, they “learn how to mean” according to the metafunctional principle of adult semiosis.