Halliday & Matthiessen (2014: 698):
The importance of modal features in the grammar of interpersonal exchanges lies in an apparent paradox on which the entire system rests – the fact that we only say we are certain when we are not. If unconsciously I consider it certain that Mary has left, I say, simply, Mary’s left. If I add a high value probability, of whatever orientation, such as Mary’s certainly left, I’m certain Mary’s left, Mary must have left, this means that I am admitting an element of doubt – which I may then try to conceal by objectifying the expression of certainty. Hence whereas the subjective metaphors, which state clearly ‘this is how I see it’, take on all values (I’m sure, I think, I don’t believe, I doubt, etc.), most of the objectifying metaphors express a ‘high’ value probability or obligation – that is, they are different ways of claiming objective certainty or necessity for something that is in fact a matter of opinion. Most of the ‘games people play’ in the daily round of interpersonal skirmishing involve metaphors of this objectifying kind.