Suppose that one wishes to construct, use, and maintain a body of information describing what is known about the state of the world. One would naturally wish to have a means of updating the information as more facts are gathered about the state of the world, and as the world undergoes change. But how can new facts be added to a body of knowledge when the new facts may contradict preexisting information? For example, given the new information that “b or c is true,” how can one get rid of all outdated information about b and c, add the new information, and yet in the process not disturb any other information in the knowledge base? The simplest choice is to place the burden on the users, by requiring them to determine exactly what to add and remove from the knowledge base. But what is really needed is a way to specify the desired change intensionally, by stating a formula that the state of the world is now known to satisfy and letting the knowledge base update algorithms automatically accomplish that change.

In this book we examine application-independent means of updating data and knowledge bases, viewing them abstractly as finite sets of sentences in propositional and first-order logic. We anticipate that an appropriate treatment of updates can be designed for a particular application by coupling the relevant application-independent techniques with additional principles and heuristics that are specific to the application domain.