What Causally Insensitive Events Tell us About Overdetermination

Suppose that Billy and Suzy each throw a rock at window, and either rock is sufficient to shatter the window. While some consider this a paradigmatic case of causal overdetermination, in which multiple cases are sufficient for an outcome, others consider it a case of joint causation, in which multiple causes are necessary to bring about an effect. Some hold that every case of overdetermination is a case of joint causation underdescribed: at a maximal level of description, every cause is necessary to bring about the outcome in precisely the way that it occurs. -/- This paper shows the latter principle to be false. I introduce a novel class of events that are insensitive to the additive force of multiple causes. They are to be contrasted with sensitive events, which physically and counterfactually vary according to the number and sorts of causes they have. I argue that sensitive effects are symptoms of joint causation; insensitive effects are symptoms of overdetermination. Insensitive effects resulting from multiple causes cannot be classified as "joint causation underdescribed," but only as overdetermination. -/- I suggest that cases of "trumping preemption" should be understood as cases of overdetermination with insensitive effects. Consequently, Lewis' influence account of causation cannot handle these cases.