Halliday & Matthiessen (1999: 558):
Modality, the speaker's angle on what is or what should be, is notoriously fluid and shifting in its categories, probably in every language. In English, there is a fairly clearly defined semantic region construed at the intersection of a number of grammatical systems, including (1) type: probability / usuality // obligation / readiness: inclination / ability; (2) value: median // high / low; (3) orientation: objective / subjective; (4) immediacy: immediate (neutral) / remote (oblique); (5) polarity: positive / negative; and one or two others. These are realised synthetically in various ways, one of which is by the modal finite operators can, could, may, might, will, would, should, must, ought-to (and one or two other fringe members).