The duress defense is available when a defendant commits a crime to prevent death or serious injury to himself or another threatened by a third party. On the other hand, the defense of necessity refers to the circumstances in which a person chooses to commit a crime to avoid a greater evil or another that would arise from objective dangers arising from the circumstances in which he finds himself. The difference between these two similar defenses is that duress is considered an excuse in English law, while necessity is considered a justificatory defense. For example, in the case of the Missouri State v Green decision, the defendant was sentenced to imprisonment for a term of three years. While serving this sentence, the defendant committed the crime of prison escape to protect himself from submission to the threatened assault. The defense of necessity is based on the principle of "necessity and not emergency". As a justificatory defense, he points out that the actor chooses an option between two "evils" and faces the lesser of them. Fundamentally, the meaning of the defense of necessity involves a balancing of evils. The crime committed by the accused must involve a lesser evil. Basically, necessity is a defense when the defendant kills one person to save the lives of many others. Brooke LJ in the Re A case stated that there are three requirements for the application of the necessity exception. First, the act is necessary to avoid inevitable and irreparable harm. Secondly, no more should be done than is reasonably necessary to achieve the objective. Finally, the harm inflicted must be proportionate to the harm avoided....... middle of the paper ......perfect on the other patient. Furthermore, Brooke LJ was also of the opinion that the doctrine of double effect might not apply in this case. The reason was that it was impossible to say that the surgeons would act in Mary's best interests when the operation would have saved Jodie but killed her. Robert Walker LJ was also told that although the operation would be in the best interests of both twins, he recognized that killing Mary simply because it was in her interests to die would be murder. He was also of the opinion that, from a legal point of view, this could not be considered anything other than a negative consequence. However, the harmful consequence would not be wrongful because it was not the motive or intention of the operation. Furthermore, common law also recognizes a narrow version of justifying necessity, which operates primarily but not entirely in the area of ​​medical assistance.