The thesaurus has been - and still is - very important in the self-images of library and information professionals and scientists. However, as indicated by the recent debate in the ISKO UK (2015) the role of the thesaurus in modern information retrieval seemingly has shrunk from what it once was (although it won the day in the final voting of this debate). Why is this the case? What is the future prospect for thesauri?
The three main points of this paper are: (1) Any knowledge organization system (KOS) is today threatened by Google-like systems, and it is therefore important to consider if there still is a need for knowledge organization (KO) in the traditional sense. (2) A thesaurus is a somewhat reduced form of KOS compared to, for example, an ontology, and its "bundling" and restricted number of semantic relations has never been justified theoretically or empirically. Which semantic relations are most fruitful is thus an open question. (3) A thesaurus is today mostly considered a standardized tool but different domains may need different kinds of KOS including different sets of relations between terms. It is urgent that progress in information science and KOS is evaluated from proper theoretical perspectives. A specific KOS is not a “perfect language” (Eco, 1995) that is able to remove the ambiguity of natural language. Much ambiguity should be understood as the battle between many “voices” (Bakhtin, 1981) or “paradigms” (Kuhn, 1962). In this perspective, a specific KOS, e.g. a thesaurus, is just one "voice" among many voices, and that voice has to demonstrate its authority and utility.