 The assumption that correlations between two variables that are not in a cause-effect relation to each other be explained by some common cause was named the principle of the common cause by Reichenbach (1956, § 19), and for this reason is often referred to as Reichenbach’s Common Cause Principle, though Reichenbach made no pretense of originating the principle, and regarded it as a codification of a mode of inference common in both science and everyday life. See the entry on  Reichenbach’s common cause principle  for more details. A variable \(C\) is a Reichenbachian common cause of a correlation between two variables \(A\) and \(B\) if the variables \(A\) and \(B\) are uncorrelated, conditional on specification of the value of \(C\). Reichenbach’s Common Cause Principle says that, if two correlated variables are not in a cause-effect relation with each other, there is a Reichenbachian common cause of their correlations. The condition we have called PLC-1 does not, by itself, entail PLC-2, but it does follow from the conjunction of PLC-1 and Reichenbach’s Common Cause Principle.