 Penrose and Percival (1962) propose a general causal principle to handle our problem. The principle states that the effects of interactions happen after those interactions but not before. Similar to Reichenbach’s principle of the common cause, they suggest what they dub the Law of Conditional Independence, namely, that “If A and B are two disjoint 4-regions, and C is any 4-region which divides the union of the pasts of A and B into two parts, one containing A and the other containing B, then A and B are conditionally independent given c. That is, Pr(a&amp;b/c) = Pr(a/c) × Pr(b/c), for all a,b.” (Penrose and Percival 1962, p. 611). Here c is an event that is a common cause that screens off the correlation between events in A and B.