R. A. Fisher was one of the most influential scientific thinkers of the twentieth century. He was mentioned earlier for his seminal contributions regarding accurate estimates of the likelihood of an error emerging from a given data set (P values, discussed in Chapter 9). Fisher appreciated that the correlation of two variables only indicated that they have some association, but could not demonstrate causality. In the twentieth century, data began to emerge that people who smoked had a higher rate of lung cancer than those who did not smoke, beginning a debate that would rage for close to a century regarding the carcinogenic effects of tobacco. Unlike most who began to develop the view that smoking tobacco probably increased one’s risk of cancer, Fisher became convinced that it was in fact the other way around; he essentially argued that cancer caused smoking.1 This view, which seems curious in retrospect, was quite logical at the time (and remains logically valid).