 Embedded in the Principle of the common cause, that Salmon borrowed from Reichenbach, screening off has the capacity to discern genuine from spurious causal links. In Reichenbach’s formulation, such principle states that “if an improbable coincidence has occurred, there must exist a common cause” (Reichenbach 1956: 157). Reichenbach named the structure underlying the principle of the common cause conjunctive fork, to be defined as follows. Take two events A and B which happen simultaneously more frequently than would be expected on the basis of pure chance. Then we have that \(p (A \amp B) \gt p (A) \times p (B)\), namely the two events are not independent. If in the presence of a third event C the correlation between A and B is absorbed, so that the two events become reciprocally independent if taken relative to C and \(\msim C\), we have a conjunctive fork. For a conjunctive fork the following relations hold: