Diagnosis: Medical diagnosis program bases on probabilistic analysis have been able to perform at the level of an expert physician tin several areas of medicine. Heckerman(1981) describes a case where a leading experts on lymph node pathology scoffs at a program diagnosis of an especially difficult case. The creators of the program suggest he ask the compute for an explanation of the diagnosis. The machines points out the major factors influencing its decision and explains the subtle interaction of several of the symptoms in this case. Eventually the expert agrees with the program.