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88 January 1999/Vol. 42, No. 1 COMMUNICATIONS OF THE ACM our environments (such as walking, talking, typing, and mouse clicking). Knowledge acquisition is based mainly on practice. Procedural knowledge never requires verbal support and is very difficult to describe but, once learned, is rarely forgotten. Such knowledge includes what we call know-how, or knowledge built up through experience. Early designers of expert systems underestimated the complexity of this knowledge concept. Declarative knowledge, based on facts, is static and concerned with the properties of objects, persons, and events and their relationships. Declarative memory contains all the information that is consciously and directly accessible. Declarative knowledge is easy to describe and to communicate. Declarative memory consists of two types of knowledge—topic, or semantic, and episodic.

Topic knowledge refers to the meaning …