Difficulty in capturing "deep" knowledge of the problem domain. MYCIN, for example, lacks any real knowledge of human physiology. It does not know what blood does or the function of the spinal cord. Folklore has it that once, when selecting a drug for treatment of meningitis, MYCIN asked whether the patient was pregnant, even though it had been told that the patient was male. Whether this actually occurred or not, it does illustrate the potential narrowness of expert systems.?