Halliday (2008: 64):
Polarity can be taken as the quintessential example of a grammatical system; it is involved in everything we say — everything that language can turn into meaning. It has a place in all metafunctions — in a sense it is pre-metafunctional; this is why it can be ambivalent, if realised on its own (as yes or no), and can be lexicalised in both ideational and interpersonal combinations (eg allow / forbid; nice / nasty). It provides a model of how meaning gets grammaticalised: no doubt it was central to the evolution of language in the species, though that we can never know; it is the first opposition to appear in the development of language in infancy, in the context of “I want / I don’t want” signalled by sound or gesture.