Commentary The rational man, said Bertrand Russell, holds his beliefs with a strength that is proportional to the evidence in their favor: to which one might add that he also fears dangers in proportion to their likelihood of their coming to pass. Unfortunately, on these definitions no rational man has ever existed or ever could exist. Even if it were possible to measure the strength of a man’s beliefs or fears on a valid and reproducible scale, the fact is that none of us either does or can spend his life examining the evidence for all that he believes or fears. At best, we can do so only intermittently and in bursts. We are obliged to take much on trust or according to our prejudices. No subject now arouses more passion than immunization against epidemic disease, as it has always done. Possibly the most popular and persistent social movement in …