Halliday & Matthiessen (2014: 367):
The word the is a specific, determinative Deictic of a peculiar kind: it means ‘the subset in question is identifiable; but this will not tell you how to identify it — the information is somewhere around, where you can recover it’. … Hence the is usually accompanied by some other element which supplies the information required; … If there is no such information supplied, the subset in question will either be obvious from the situation, or else will have been referred to already in the discourse …