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In this article we use the term “object” to mean any individual thing. If an object is currently in a person’s view, the person may refer to that object simply by ostension (pointing at the object). Whether or not an object is in view, one may refer to it by using a linguistic expression. This allows one to reference concrete objects from the past (e.g. Einstein), the present (e.g. this article), or the future (e.g. the next solar eclipse), as well as intangible objects (e.g. a specific course in linguistics).
An information system models a specific universe of discourse (UoD), also known as a business domain, or world of facts about which users wish to discourse within the business. For example, one UoD might be concerned with a company’s product sales and orders, and another UoD might deal with hotel bookings. In natural language, linguistic expressions used to reference objects within a given UoD are typically proper names (e.g. “Barack Obama”) or definite descriptions (e.g. “the president of the USA”) (Allen, 1995).
In philosophy, many different proposals exist regarding the precise nature of proper names (e.g. see http://plato.stanford.edu/entries/rigid-designators/). The term “possible world” may be assigned different meanings. In this article, a possible world is treated as a state of the UoD being modeled by an information system, and proper names are treated as rigid identifiers within the UoD of interest. Definite descriptions are often characterized as non-rigid, since some of them may refer to different objects in different possible worlds. For example, if we take “the president of the USA” as shorthand for “the current president of the USA”, then uttering this expression in 2003 refers to George W. Bush, while uttering the same expression in 2013 refers to Barack Obama—a simple example of deixis where the denotation of a term depends on its context (in this case the time of utterance). However, given our sense of possible world, some definite descriptions are rigid designators (within a given UoD). For example, if we restrict the UoD to this world history, the definite description “the 44th president of the USA” always refers to Barack Obama. Moreover, if we further restrict the UoD to the year 2013 then “the president of the USA” is a rigid designator within that UoD.