Halliday & Matthiessen (1999: 529):
From a 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 a 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.