 Hitchcock (2001) presents a useful regimentation of this reasoning. He defines a route between two variables X and Z in the set V to be an ordered sequence of variables &lt;X, Y1,…, Yn, Z&gt; such that each variable in the sequence is in V and is a parent of its successor in the sequence. A variable Y (distinct from X and Z) is intermediate between X and Z if and only if it belongs to some route between X and Z. Then he introduces the new concept of an active causal route: