ABSTRACT
Imagine you and I both throw a brick at a window, where both bricks pass through it simultaneously. Each brick throwing is a distinct event, but both appear to be causes of the window shattering – it is not that one caused it and the other did not, for they both have as much right as the other to count as a cause. However, each is a redundant cause of the window shattering. If the first brick had not thrown, the second brick would have shattered the window, and vice versa. This is a case of overdetermination (sometimes called ‘symmetric overdetermination’): each of the two causes has an equal claim to being a cause of the effect. (It contrasts with pre-emption, another form of redundant causation, where only one of the potential causes has a claim to being a cause of the effect, as when the washing dries because you hung it out, but if you had not hung it out someone else would have done – so it would still have got dry.)
Cases of overdetermination have been suggested as counter-examples to the counterfactual theory of causation. If the event of throwing the first brick had not occurred then the window would still have shattered, so the window shattering does not counterfactually depend in the appropriate manner upon the first brick being thrown – therefore, according to the naïve version of the counterfactual theory, it does not cause it either. Nor does it do so according to the more sophisticated version described in pre-emption: there is no ‘chain’ of causal dependence running from either of our putative causes to the effect, so, again we get the wrong answer that neither of them are causes. Hence, nobody shattered the window! Hence, at least, goes the problem.
