Halliday & Matthiessen (2014: 691):
The reason this area of the semantic system is so highly elaborated metaphorically is to be found in the nature of modality itself. … Modality refers to the area of meaning that lies between yes and no – the intermediate ground between positive and negative polarity. What this implies more specifically will depend on the underlying speech function of the clause. (1) If the clause is an ‘information’ clause (a proposition, congruently realised as indicative), this means either (i) ‘either yes or no’, i.e. ‘maybe’; or (ii) ‘both yes and no’, i.e. ‘sometimes’; in other words, some degree of probability or of usuality. (2) If the clause is a ‘goods-&-services’ clause (a proposal, which has no real congruent form in the grammar, but by default we can characterise it as imperative), it means either (i) ‘is wanted to’, related to a command, or (ii) ‘wants to’, related to an offer; in other words, some degree of obligation or of inclination. We refer to type (1) as MODALISATION and to type (2) as MODULATION; this gives a system as in Figure 10-5.
