 Skyrms proposed a slightly weaker condition: a cause must raise the probability of its effect in at least one background context, and lower it in none. A background context is a conjunction of factors. When such a conjunction of factors is conditioned on, those factors are said to be “held fixed”. To specify what the background contexts will be, then, we must specify what factors are to be held fixed. In the previous example, we saw that the true causal relevance of smoking for lung cancer was revealed when we held country living fixed, either positively (conditioning on \(B)\) or negatively (conditioning on \({\nsim}B)\). This suggests that in evaluating the causal relevance of C for E, we need to hold fixed other causes of E, either positively or negatively. This suggestion is not entirely correct, however. Let C and E be smoking and lung cancer, respectively. Suppose D is a causal intermediary, say the presence of tar in the lungs. If C causes E exclusively via D, then D will screen C off from E: given the presence (absence) of tar in the lungs, the probability of lung cancer is not affected by whether the tar got there by smoking. Thus we will not want to hold fixed any causes of E that are themselves caused by C. Let us call the set of all factors that are causes of E, but are not caused by C, the set of independent causes of E. A background context for C and E will then be a maximal conjunction, each of whose conjuncts is either an independent cause of E, or the negation of an independent cause of E.