Halliday & Matthiessen (2014: 176):
Polarity is thus a choice between yes and no. But these are not the only possibilities; there are intermediate degrees, various kinds of indeterminacy that fall in between like ‘sometimes’ or ‘maybe’. These intermediate degrees, between the positive and negative poles, are known collectively as MODALITY. What the modality system does is to construe the region of uncertainty that lies between ‘yes’ and ‘no’.