 Reichenbach’s definition of causation, discussed in  Section 2.2  above, appeals to time order: it requires that a cause occur earlier than its effect. But Reichenbach also thought that the direction from causes to effects can be identified with a pervasive statistical asymmetry. Suppose that events A and B are correlated, and that C satisfies conditions 2–6 above, so that ABC form a conjunctive fork. If C occurs earlier than A and B, and there is no event satisfying 2 through 6 that occurs later than A and B, then ACB is said to form a conjunctive fork open to the future. Analogously, if there is a later event satisfying 2 through 6, but no earlier event, we have a conjunctive fork open to the past. If an earlier event C and a later event D both satisfy 2 through 6, then ACBD forms a closed fork. Reichenbach’s proposal was that the direction from cause to effect is the direction in which open forks predominate. In our world, there are a great many forks open to the future, few or none open to the past. However, we shall see in  section 3.6  below that conjunctive forks are not the best structures for identifying causal direction. 