Polarity And Modality

Halliday & Matthiessen (1999: 525-6):

The mood system constructs the clause as a move in an argument: either as a "proposition" (statement or question) or as a "proposal" (offer or command). The system provides scope for argument by incorporating an opposition of 'on' or 'off': each clause assigns either positive or negative polarity. Every proposal or proposition selects one or the other: either that was a snake or that wasn't a snake, either catch it! or don't catch it! But at the same time the interpersonal grammar goes much further, it rejects a simple polarity of 'yes' and 'no', opening up a broad semantic space in between. This is the area of "modality", where the interactants present different aspects of their own judgments and opinions, exploring the validity of what is being said and typically locating it somewhere between the positive and negative poles.