Modal Responsibility: Propositions

Halliday & Matthiessen (2014: 146):

This rôle [modal responsibility] is clearly recognisable in the case of offers and commands; but it is the same principle that is at work in statements and questions. Here, too, the Subject specifies the ‘responsible’ element; but in a proposition this means the one on which the validity of the information is made to rest. (It is important to express it in these terms rather than in terms of true or false. The relevant concept is that of exchangeability, setting something up so that it can be caught, returned, smashed, lobbed back, etc. Semantics has nothing to do with truth; it is concerned with consensus about validity, and consensus is negotiated in dialogue.)