 (We are oversimplifying, and treating asbestos and lung cancer as binary variables.) We observe that Ted was in fact exposed to asbestos and did in fact develop lung cancer. What is the probability of the counterfactual: “If Ted had not been exposed to asbestos, he would not have developed lung cancer”? Pearl (2009) calls a probability of this form a probability of necessity. It is often called the probability of causation, although this terminology is misleading for reasons discussed by Greenland and Robins (1988). This quantity is often of interest in tort law. Suppose that Ted sues his employer for damages related to his lung cancer. He would have to persuade a jury that his exposure to asbestos caused his lung cancer. American civil law requires a “more probable than not” standard of proof, and it employs a “but for” or counterfactual definition of causation. Hence Ted must convince the jury that it is more probable than not that he would not have developed lung cancer if he had not been exposed.