An expert system is a computer system emulating the decision-making ability of a human expert. Expert systems were among the first truly successful forms of [[artificial intelligence]]. Expert systems typically represent logic in the form of if-then statements and knowledge in the form of a knowledge base. The system can then reason through a body of knowledge in much the same way a human would. Diagnostic systems for medicine and biology were early targets of expert systems. Expert systems gave us the terminology still in use today where AI systems are divided into a [[knowledge base]], which includes facts and rules about a problem domain, and an [[inference engine]], which applies the knowledge in the knowledge base to answer questions and solve problems in the domain. Expert systems have now been folded into the more broad **rule-based system** category.