The basic message of an imperative clause is ʻI want you to do somethingʼ, or ʻI want us (you and me) to do somethingʼ [...] What of the common everyday form of the ʻyouʼ imperative, which has no Subject or finite verb, as in 'Keep quiet, sing a song of sixpence?' Strictly speaking, these have no explicit Theme; the meaning ʻI want you toʼ, which might have been thematized, by analogy with those above, or with the interrogative, is realized simply by the form of the clause. Structurally, therefore, these imperatives may be considered as consisting of Rheme only, the thematic component of request being left implicit.
- Halliday (1985), Pag. 49