he showed that if we knew the causal quantities in Figure 2.7, we could predict correlations in the data (not shown in the diagram) by a simple graphical rule. This rule sets up a bridge from the deep, hidden world of causation to the surface world of correlations. It was the first bridge ever built between causality and probability, the first crossing of the barrier between rung two and rung one on the Ladder of Causation.
