expert system, a computer system or program that uses artificial intelligence techniques to solve problems that ordinarily require a knowledgeable human. The method used to construct such systems, knowledge engineering, extracts a set of rules and data from an expert or experts through extensive questioning. This material is then organized in a format suitable for representation in a computer and a set of tools for inquiry, manipulation, and response is applied. While such systems do not often replace the human experts, they can serve as useful adjuncts or assistants. Among some of the successful expert systems that have been developed are INTERNIST, a medical diagnosis tool that contains nearly 100,000 relationships between symptoms and diseases, and PROSPECTOR, an aid to geologists in interpreting mineral data.