EMYCIN's CF calculations allow contradictory rules to cancel one another. While in logic a contradiction is intolerable, use of rules and exceptions, expressed as contradicting rules, seems to be the way humans often think.
Example: FUO program for diagnosing fever of unknown origin. Patient is a 17 year old female with persistent high fever, headache, lethargy, cardiac symptoms....
Results: Lung cancer (.81), Endocarditis (.7)
Endocarditis was the correct diagnosis. The physician expert remarked that lung cancer was consistent with the symptoms, but that patients that young never get lung cancer.
One way to handle this in EMYCIN is to add a rule that ``rules out'' certain cancers in young patients; the CF can be made a function of the patient's age.