 According to Cartwright, the set \(\b{K}\) should not include variables that are causally intermediate between \(C\) and \(E\). Such variables are called mediators. To see why, imagine a drug reduces the risk of heart disease by producing a chemical, represented by variable \(Z\), in the blood stream, and via no other factors. If \(C\) and \(E\) have no common causes, they will be probabilistically independent conditional on \(Z\). Intuitively, one should not hold the blood chemical fixed in evaluating the effect, since it is the means by which the effect is brought about.