The Book of Why: The New Science of Cause and Effect

by Judea Pearl

At the first level, association, we are looking for regularities in observations. This is what an owl does when observing how a rat moves and figuring out where the rodent is likely to be a moment later, and it is what a computer Go program does when it studies a database of millions of Go games so that it can figure out which moves are associated with a higher percentage of wins. We say that one event is associated with another if observing one changes the likelihood of observing the other. The first rung of the ladder calls for predictions based on passive observations. It is characterized by the question “What if I see …?” For instance, imagine a marketing director at a department store who asks, “How likely is a customer who bought toothpaste to also buy dental floss?”