Earlier this week I wrote of sequence and causation. If a city experiences both an increase in the crime rate and an increase in unemployment, what can we infer? If you are of one political persuasion, you will surely infer that the criminals drove businesses out of town, leading to an increase in unemployment. If you are of another political persuasion, you’ll as readily declare that the increase in unemployment made people desperate for money, or angry, or both, and led to the higher level of crime. But that is polemics, not fact finding. Perhaps the truth of the matter is a bit of both, perhaps it is neither. The correlation doesn’t tell you. Here is an anti-Humean corollary. Even if you have a temporal sequence you can’t securely infer causation. It is true that in economics especially a reliable forecasting of event A by event B, is sometimes called “Granger causality,” after Clive Granger. But Granger causality isn’t old-fashioned, one-pool-ball-hits-another, no...