 The Common Cause Principle was introduced by Hans Reichenbach, in The Direction of Time, which was published posthumously in 1956. Suppose that two events A and B are positively correlated: \(p(A\cap B)&gt;p(A)p(B)\). Suppose, moreover, that neither event is a cause of the other. Then, Reichenbach’s Common Cause Principle (RCCP) states that A and B will have a common cause that renders them conditionally independent. Reichenbach incorporated his RCCP into a new probablistic theory of causation, and used it to describe a (purported) macrostatistical temporal asymmetry in analogy with the second law of thermodynamics. The principle is significant because it posits a connection between causal structure and probabilistic correlations, thus facilitating causal inference from observed correlations. However, RCCP has been controversial, and a number of counterexamples have been proposed. RCCP is often seen as an antecedent of the Causal Markov Condition, which plays a central role in causal modeling and causal inference. RCCP has also been taken to capture assumptions about the behavior of classical systems that appear to be violated in quantum mechanics.