 Recall that the dot is conjunction, and P(A2, A3) in Reichenbach’s notation is P(A3 | A2) = P(A3. A2) / P(A2) in the currently conventional notation. Reichenbach goes on to claim that a common effect of two causes is causally between its two causes, thus A1 → A2 ← A3, which is obviously false if absence of any causal connection implies independence. Throughout his life, Reichenbach implicitly makes this assumption in his examples about causal relations. With such an assumption, it follows that P(A1. A3) = P(A1) P(A3) ≥ P(A1, A3) = P(A1)P(A3)/P(A3) = P(A1), contradicting (1) in the definition. One may take these and other examples as evidence that Reichenbach did not have in mind instances of what is now called the causal Markov condition—that conditional on its relatively direct causes a variable is independent of variables that are not its effects—or, alternatively, that he was simply confused about the properties of “colliders”—common effects—in causal graphs. We prefer the latter interpretation. 