Martin (1992: 461):
The concept of modal responsibility is less obvious in propositions, and the meaning of Subject is hard to isolate because of the fact that in English declaratives and interrogatives Subject conflates with unmarked topical Theme. However, it is clear in contexts where marked Themes are used to scaffold a text's method of development that Subject selection is in principle independent of Theme selection.
 This is not true. The concept of modal responsibility is no less obvious in propositions. In propositions, modal responsibility is responsibility for the validity of what is being stated or questioned (rather than offered or commanded, as in proposals).
 The meaning of Subject is modal responsibility. The identity of Subject is "isolated" by Halliday & Matthiessen (2014: 149) as follows:
In fact the Subject in English has got a distinct identity, as we have pointed out; its identity can be established if we adopt a trinocular perspective, as suggested by the stratificational model of language.
(i) From below, it is that nominal element (nominal group or nominalised phrase or clause) that is picked up by the pronoun in the mood tag.
(ii) From round about, it is that which combines with the Finite (operator) to form the Mood element in the clause; it is also that which constitutes the unmarked Theme if the mood is declarative, and which switches place with the Finite if the mood is yes/no interrogative.
(iii) From above, it is that which carries the modal responsibility: that is, responsibility for the validity of what is being predicated (stated, questioned, commanded or offered) in the clause.
 The conflation of Subject with Theme in the unmarked case in declarative clauses has no bearing on the meaning or identity of Subject. Subject is an interpersonal function (modal responsibility of the clause as exchange), whereas Theme is a textual function (point of departure of the clause as message).
 Subject does not conflate with unmarked topical Theme in declarative clauses. This wording betrays Martin's misunderstanding of un/marked Theme. If Theme conflates with Subject in declarative clauses, then the Theme is said to be unmarked (whereas if it conflates with Adjunct or Complement, it is said to be marked). Halliday & Matthiessen (2004: 73, 74):
In a declarative clause, the typical pattern is one in which Theme is conflated with Subject; … We shall refer to the mapping of Theme on to Subject as the unmarked Theme of a declarative clause. The Subject is the element that is chosen as Theme unless there is good reason for choosing something else.
 Whatever the context, Subject selection is independent of Theme selection — regardless of whether the functions are conflated or not. Subject selection is an interpersonal choice — one which identifies where the responsibility for the validity of the predication lies. Theme selection is a textual choice — one which identifies the element that is highlighted as the point of departure for what follows in the message.A Theme that is something other than the Subject, in a declarative clause, we shall refer to as a marked Theme. The most usual form of marked Theme is an adverbial group … or prepositional phrase … functioning as Adjunct in the clause. Least likely to be thematic is a Complement, which is a nominal group that is not functioning as Subject — something that could have been a Subject but is not … . Sometimes even the Complement from within a prepositional phrase functions as Theme … .