In U.S.
criminal law,
necessity may be either a possible
justification or an exculpation for breaking the
law.
Defendants seeking to rely on this
defense argue that they should not be held
liable for their
actions as a
crime because their conduct was
necessary to prevent some greater harm and when that conduct is not excused under some other more specific provision of law such as
self defense. Except for a few statutory exemptions and in some medical cases there is no corresponding defense in English law for murder.