Halliday & Matthiessen (1999: 241-2):
In a similar way, phenomena in the ideation base also map onto constructs of interpersonal meaning. For example, a congruent figure maps onto a move in a dialogic exchange; it is enacted interpersonally as a proposition or a proposal. It follows that when phenomena are reconstrued metaphorically within the ideation base, there are also interpersonal consequences. For instance, the figure 'atomic nucleus + absorb + energy' can be enacted interpersonally as a proposition that is open to negotiation: The atomic nucleus absorbs energy — Does it? — Yes, it does — No, it can't.
However, when this figure is reconstrued as the participant 'absorption (+ of energy) (+ by atomic nucleus)', it no longer has the potential for being enacted interpersonally as proposition; rather, it would be taken for granted in discourse. You can't argue with the absorption of energy by the nucleus since it is not enacted as an arguable proposition. Such interpersonal differences can have a powerful rhetorical effect in persuasive discourse. (There is an analogous effect with respect to proposals in regulatory discourse.)