ABSTRACT
Imagine I place a bomb on a bridge in just the right place so that, when detonated, the bridge collapses. When the timer reaches zero, the bomb explodes destroying the bridge. The timer reaching zero seems to be a cause of the bridge collapsing. Also, my placing the bomb seems to have been a cause of the bridge collapsing. Neither cause, however, is sufficient for the effect to take place. If I had placed the bomb correctly but there was no timer to set it off, the bridge would have remained intact. If the timer had reached zero but I had not bothered to place it on the bridge, instead staying at home in bed, then the bridge would have remained intact (and if I would stored the bomb at home, I would have had a rude awakening). Nor are these causes necessary – the bridge could have collapsed without either cause taking place, for instance if it had become rusted, or someone had loaded heavy weights on it until it collapsed, or someone else had bombed it.
