A duty to prevent violence finds support not only in the Anglo-American common-law
tradition but also in liberal political theory. According to Locke and other natural rights
theorists, individuals enter into society to preserve their lives, liberties, and properties.
Under the social contract, citizens obtain a right to protection by the community
against criminal violence. In return, they promise not only to comply with the laws,
but also to assist the authorities in enforcing those laws. In this way, Locke writes,
the rights of individuals come to be defended by “the united strength of the whole
Society.” In On Liberty, John Stuart Mill recognizes a similar duty on the part of individuals.…
Mill agrees “that everyone who receives the protection of society owes a
return for the benefit,” including an obligation to bear one’s fair share of “the labours
and sacrifices incurred for defending the society or its members from injury.”