 That is, the probability that both A and B occur is greater than the product of the individual probabilities. Reichenbach’s Common Cause Principle says that when such a probabilistic correlation between A and B exists, this is because one of the following causal relations exists: A is a cause of B; B is a cause of A; or A and B are both caused by a third factor, C. In the last case, the common cause C occurs prior to A and B, and must satisfy the following four independent conditions: 