Halliday & Matthiessen (2014: 707):
We have explored certain key strategies involved in the metaphorical expansion of the interpersonal meaning potential, showing that they are not random or ad hoc features of the system but rather motivated and principled extensions of the congruent system. There are, of course, other strategies as well. The lexicogrammatical resources of MOOD, and the associated patterns of MODALITY and KEY, carry a very considerable semantic load as the expression of interpersonal rhetoric. Not surprisingly, these categories lend themselves to a rich variety of metaphorical devices; and it is by no means easy to decide what are metaphorical and what are congruent forms. Some common speech-functional formulae are clearly metaphorical in origin, for example
(i) I wouldn’t ... if I was you: command, congruently don’t ... ! functioning as warning;(ii) I’ve a good mind to ... : modalised offer, congruently maybe I’ll ..., typically functioning as threat;(iii) she’d better ... : modulated command, congruently she should ... , typically functioning as advice.
Some words, such as mind, seem particularly to lend themselves to this kind of transference: cf. would you mind ... ?, mind you!, I don’t mind ... (including I don’t mind if I do, positive response to offer of drink in the environment of a pub), and so on.