Expert Systems are based on the assumption that humans are good problem solvers because they have a large amount of knowledge, and that by giving all of this knowledge to a computer it can act as an expert in a particular subject. An Expert System uses rules that the programmer gives it to "reason" about a subject. These rules are found through researching what experts on the subject have printed, or by interviewing them directly and then compiling all the facts.
     Expert Systems are made up of a knowledge base and an inference engine. The inference engine uses the rules in the knowledge base to reason about a given subject, much like a doctor would use his knowledge of the human body to diagnose a patient.