Most semantic theories distinguish what we will call purely denotational expressions ("man, blue, walk, day, bachelor") from indexical or deictic expressions ("he, over there, yesterday", the "bachelor"). [...] For an expression to be indexical, however, it must have a fixed sense and denotation, but a shifting reference [Bar-Hillel, Yehoshua, 1954, Indexical expressions, Mind 63, pp. 359-379]. "He", for example, has a fixed sense, say "male person", and denotes male people in every real or imaginary world. But the particular person it refers to - its referent - changes with the time, places and circumstances of its utterance.
- Clark & Clark (2004), a pag.147-148