Halliday & Matthiessen (1999: 530-1):
Reference is a way of referring to things that are already semiotically accessible: either actually, in the text, or potentially, in the context of situation. The English reference systems are the personals, especially the third person pronouns and determiners he/him/his she/her/hers it/its they/them/their/theirs, and the demonstratives this/these that/those and the maverick the (which emerged as a weakened form of that). Such systems evolved in a deictic function; when used anaphorically or cataphorically (that is, in deictic relation to text), they create cohesion. There is also a third source of referential cohesion, through the use of comparison, with words such as same, other, different, less, smaller.