Analyze each of the following reports, explaining the way in which the method of agreement and the method of difference have been jointly applied, and identifying the special force, if any, of their combination.

Pain can be agonizing, but it serves a useful function: It teaches people and animals to avoid dangers, and forces them to attend to wounds. Strangely, there are a very few people who never feel pain; they remain unaware of having suffered significant injuries. One family in northern Pakistan has several such members. One, a ten-year old boy, became famous for giving street performances in which he put knives through his arms and walked on hot coals. Tissue damage would result, but no discomfort. Geneticist C. G. Woods of Cambridge University searched for the cause of this remarkable inability to feel pain. Eventually he zeroed in on mutations in a gene, SCN9A, that codes for the channel through which sodium enters pain-sensing cells, critical to the pain signal. Testing with electric current, he could open and close sodium channels on some cells—but he could not open the sodium channels on those mutant cells. Said Woods, “This shows that rare diseases can still be of great importance because of the insights they give into biological processes.” A Yale University neurologist, Stephen Waxman, observes that if researchers could craft a drug that can make these channels inactive, as they are in the Pakistani family members, millions of people worldwide who suffer from chronic pain would be wonderfully served.

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