Halliday & Matthiessen (1999: 529):
But the Theme + Rheme configuration becomes a message only when paired with another one, that of Given + New. This construes a piece of information from the complementary point of view, as something having news value — something the listener is being invited to attend to. It may not contain anything the listener has not heard before; a great deal of ”news” is totally familiar, being simply contrasted or even re-iterated. On the other hand, the entire message may consist of unknown information, for example the first clause in a piece of fiction. But the message is construed along prototypical lines as an equilibrium of the given and the new, with a climax in the form of a focal point of information: ‘this is to be the focus of your attention’.