6 Causation, Neo-Humean approaches

一个疑难，出现于对INUS condition作为一种对“原因”概念的分析的一节中：
Lewis mentions another problem for an account like Mackie’s. This is the problem of causal preemption. Here, we have two events, a and b, each of which, taken by itself, would cause a third event, c. However, when a and b both occur, a acts to block b’s normal causal role and goes on to cause c all by itself.
a→c
↘
b ||
So b occurs, but is preempted by a. However, since the circumstances are such that had a not blocked b, b would have caused c, b is an insufficient, but necessary component in a bundle of factors (including b and the relevant circumstances) that while sufficient for c is not necessary for c. So b is an INUS condition for c; but it is not the cause of c, so, again, a counterexample to Mackie’s analysis.

不懂b怎么就算一个necessary component了啊。。。

necessary condition的定义（P.197）：
Thus, he proposes that we understand the claim
(1) Event x was a necessary condition for event y
in terms of the counterfactual conditional
(2) If x had not occurred, y would not have occurred;

可是如上例，b如果不出现，c仍然会出现啊。。。

我怀疑其实是作者写错了，这里的反例其实是a：In this counterexample to Mackie's analysis, a alone is the cause of c. However, since the circumstances are such that had a not occurred, c would have occurred anyway because b would have caused c, a is not a necessary component in this bundle of factors, thus a is not an INUS condition.