The term “index” was first used by Peirce, who defines it as the connection between a sign and its object, i.e., the sign points to its object. According to Levinson (1983, p. 57), Peirce “argued that they determined a referent by an existential relation between sign and referent (see Burks, 1949),” however they “have not been put to much effective use in linguistic pragmatics”[thanks Tatjana!]. Enter Michael! Who “created a
framework in which this concept could usefully become central
to understanding how language and other communicative codes
are contextualized/contextualizing” (personal correspondence).