The concept of deixis
The reference of certain kinds of expression is determined in relation to features of the utterance-act: the time, the place, and the participants, i.e. those with the role of speaker or addressee. This phenomenon is known as deixis and the expressions concerned are called deictic. Examples of such expressions are given in:
 i I bought a new stereo system yesterday.
ii Do you feel any pain here now?
iii Could you pick this up and put it with those boxes, please?
Yesterday in [1i] and now in [ii] are interpreted in relation to the time of the utterance: now refers to a time including that of the utterance, while yesterday refers to the day before that on which the utterance takes place. This is a matter of temporal deixis.
Here in [1ii] refers to a location close to the speaker, and in [iii] this refers to something located close to the speaker and those boxes to boxes that are further away. This is locative deixis.
Finally, I in [1i] refers to the speaker, and you in [ii–iii] to the addressee. This is called person deixis.
It will be recalled that we are using such terms as ‘utterance-act’ and ‘speaker’ to cover written as well as spoken language. Example [1i] could be used with the same interpretation as readily in writing as in speech; the others are unlikely to be used in writing because the reference of here, this, and those boxes is determined by very local features of the utterance-act, whereas it is characteristic of writing that writer and addressee are in different places. But of course they don't have to be: if I had lost my voice, for example, I might communicate [ii–iii] to you in writing in your presence.
◼ Varying reference
It follows from what we have said that the reference of deictic expressions potentially varies from one utterance to another. This of course applies to referring expressions generally: different utterances of Where's Kim? potentially involve reference to different people called Kim.