Contrary to the conventional view, there is strong evidence that, for centuries,
the common law of England and America did recognize an individual duty to act in
precisely such cases. According to traditional legal doctrine, every person was entitled to protection by the government against violence and injury. In return for this
protection, individuals had an obligation not merely to obey the law, but also, when
necessary, to actively help enforce it.… Thus, individuals at the scene of a violent
crime had a duty to intervene if they could do so without danger to themselves. If
they could not, they were required to notify the authorities.