Halliday (2008: 180n):
polarity is as it were pre-metafunctional: it develops (and probably evolved) prior to the distinction between ideational and interpersonal meanings. (Rather in the same way that, in the physical world, positively and negatively charged particles seem to be prior to the distinction between matter and information!) It may be that the “energy levels” of modality (high / median / low) are also of this kind: both modality and polarity are regularly grammaticalised in combination with other features.