Halliday & Matthiessen (1999: 526):
The former [deriving from the polarity of propositions] are along the dimensions of "probability" and "usuality", the first being the more elaborated of the two (because more arguable); for example,That was a snake.— It wasn't. It can't possibly have been a snake.— Couldn't it? Don't you think so? I think it might have been.— It probably wasn't. But snakes can appear round here.All of these represent probability except the final can which means 'sometimes' (usuality). Note that it is always the judgment of speaker or listener that is represented as a choice of modality, not that of any third party (this is one of the boundaries drawn between 'me-&-you' and 'the rest').