Halliday & Matthiessen (2014: 146):
It is the [Subject], in other words, in whom is vested the success or failure of the proposition. [It] is the one that is, so to speak, being held responsible — responsible for the functioning of the clause as an interactive event. The speaker rests his case on the [Subject + Finite], and this is what the listener is called on to acknowledge.
Be aware that, in his model of discourse semantics, Martin (1992: 461-91) completely misunderstands the notion of modal responsibility (evidence here), misconstruing it as an interaction pattern between strata (misconstrued as 'modules').