The Book of Why: The New Science of Cause and Effect (Penguin Science)

by Judea Pearl

Our simple coin-flip experiment proves that Reichenbach’s dictum was too strong, because it neglects to account for the process by which observations are selected. There was no common cause of the outcome of the two coins, and neither coin communicated its result to the other. Nevertheless, the outcomes on our list were correlated. Reichenbach’s error was his failure to consider collider structures—the structure behind the data selection. The mistake was particularly illuminating because it pinpoints the exact flaw in the wiring of our brains. We live our lives as if the common cause principle were true. Whenever we see patterns, we look for a causal explanation. In fact, we hunger for an explanation, in terms of stable mechanisms that lie outside the data. The most satisfying kind of explanation is direct causation: X causes Y. When that fails, finding a common cause of X and Y will usually satisfy us. By comparison, colliders are too ethereal to satisfy our causal appetites. We still want to know the mechanism through which the two coins coordinate their behavior. The answer is a crushing disappointment. They do not communicate at all.