 Now suppose that we have two events A and B, and a probability distribution p over the four states \(AB\), \(A\overline{B}\), \(\overline{A}B\), \(\overline{A} \overline{B}\). We can apply the formula for entropy to this probability distribution; holding the probability of A and B fixed, the entropy will be highest when A and B are probabilistically independent. Thus, when we find a correlation between A and B, this is like finding a system in a state of low entropy, and we should look for some event in the past of A and B to explain this correlation. This is only a formal analogy, but Reichenbach attempted to show that the same basic principles that give rise to the second law of thermodynamics would also give rise to the Common Cause Principle. For more discussion, see the entry on  thermodynamic asymmetry in time.