Suppose that Adam runs a stop sign and hits Eve, who happened to be driving through the same intersection. Who caused the accident?

If Adam were not there, the accident would have not happened. So it seems natural to say that he was the cause. But it is equally true that if Eve were not there, the accident could not have happened either. So, in a sense, Eve caused the accident as well. Adam might have been negligent, but if either party were absent, there would have been no accident.

More generally, the more people on the road, the more accidents will occur, at least up to the point of total gridlock. Even adding an obsessively safe driver to the road could increase total accident costs, since reckless drivers like Adam would now have one more car to run into.

When Eve decided whether to buy a car, she was forced to consider the average costs of accidents in which she could be involved since she had to pay for automobile insurance, and insurance rates by necessity reflect the average costs of accidents. But her insurance premiums did not include the costs that other drivers might incur by running into her.