The term 'shifter' was introduced into linguistics by linguist Otto Jespersen to refer to those elements in language whose general meaning cannot be defined without reference to the message (to describe a class of words whose meaning varies according to their situation or whose references varies).
For Jakobson, a shifter is a term whose meaning cannot be determined without referring to the message that is being communicated between a sender and a receiver.
Personal pronouns are shifters: the word 'I' designates both the speaker or sender who says 'I' and the 'I' contained in the message that is sent.
For example the pronouns 'I' and 'you', as well as words like 'here' and 'now', and the tenses, can only be understood by reference to the context in which they are uttered.
Roman Jakobson developed the concept in an article published in 1957.
Before this article, "the peculiarity of the personal pronoun and other shifters was often believed to consist in the lack of a single, constant, general meaning."
Jakobson argues that shifters do have a single general meaning; for example the personal pronoun I always means "the person uttering I".
This makes the shifter a 'symbol'.
Jakobson concludes that shifters combine both symbolic and indexical functions and "belong therefore to the class of indexical symbols."
In this way, Jakobson questions the possibility of a context-free grammar, since the enunciation is encoded in the statement itself.
Also, since grammar is implicated in parole, the langue/parole distinction is put into question.
Following Jakobson, Lacan uses the term 'shifter' (in English) to show the problematic and undecidable nature of the 'I' (Je).
However, while Jakobson defines the shifter as an indexical symbol, Lacan defines it as an indexical signifier.
This problematises the distinction between enunciation and statement.
On the one hand, as a signifier it is clearly part of the statement.
On the other hand, as an index it is clearly part of the enunciation.
This division of the 'I' is not merely illustrative of the splitting of the subject; it is that split.
"Indeed, the I of the enunciation is not the same as the I of the statement, that is to say, the shifter which, in the statement, designates him."