[...] experential Epithets are potentially defining, whereas interpersonal ones are not. Take the example of 'long' in 'long train'. If I say 'a long train', you cannot tell which particular train I am talking about, because the Deictic 'a' is non-specific; but if I say 'the long train', the specific Deictic 'the' indicates that you can tell, and that the necessary information is contained in the experential Epithets 'long'. This particular train, in other words, is defined by its length, relative to some norm – perhaps some other train or trains that are present in the context.
- Halliday (1985), Pag. 163