Halliday & Matthiessen (1999: 529):
From the speaker's point of view, a piece of information has a specific point of departure; the Prague scholars referred to this as the "Theme". The Theme, in English, always includes one element that has an experiential function, typically a participant in the process; it may include other elements as well, for example an interpersonal expression of modality if the speaker is thematising his/ her own point of view. One way of signalling what is thematic is by putting it first in the clause, as is done in English, where everything up to and including the first experiential element constitutes the speaker's chosen point of departure; for example But surely time is defined as that which you can't turn back?, where the Theme is but surely time. Here the speaker is construing a message around the theme of 'what I'm saying is contrary to what went before' (but); 'it's my opinion, and I'd like to challenge you with it' (surely); and 'the starting point is the topic of time'. The remainder of the clause constitutes the body of the message, labelled grammatically as the "Rheme".