"An information system characterizes a view of the world with which it interacts, and broadly speaking, its input can take two forms; a query or an impetus for change. Physically the information held by an information system might be a diagram, a graph, a spreadsheet, a database, a rulebase, or a more sophisticated cognitive entity. More often than not, information is uncertain and subject to change; this is the case even for simple database systems. Consequently an information system requires a mechanism for modifying its view as more information about the world is acquired."
– Mary-Anne Williams. Tutorial: Belief Revision: Modeling The Dynamics Of Information Systems, 1995.