Halliday & Matthiessen (2014: 696):
There is one further category that needs to be taken into account, that of ability/potentiality, as in she can keep the whole audience enthralled. This is on the fringe of the modality system. It has the different orientations of subjective (implicit only) realised by can/can’t, objective implicit by be able to, and objective explicit by it is possible (for ... ) to. In the last of these, the typical meaning is ‘potentiality’, as in it was possible for a layer of ice to form. In the subjective it is closer to inclination; we could recognise a general category of ‘readiness’, having ‘inclination’ and ‘ability’ as subcategories at one end of the scale (can/is able to as ‘low’-value variants of will/is willing to). In any case can in this sense is untypical of the modal operators: it is the only case where the oblique form functions as a simple past, as in I couldn’t read that before; now with my new glasses I can.