All offences are made up of two elements, that is the actus reus and the mens rea. Under Criminal Law, if the prosecution has been able to prove the actus reus and mens rea of the offence, the defendant may be guilty unless he or she is able to establish a defence.  Among all the defences, some are partial defences which do not lead to a complete acquittal, but only reduce the charge for which the defendant is convicted. It is for the prosecution to prove that the defendant was not relying on the defence if a defence is regarded as a denial of actus reus or mens rea.
 In the theory of criminal law, it is common to distinguish between justification defences and excuse defences. In the context of justifications, a defence is allowed because the special circumstances in which the action is performed make the action desirable, or at least permissible, from a social and moral point of view. Justification functions as an exception to the criminal prohibition where the action is not wrongful and is warranted.  Excuses are often to be about removing blame; that the defendant behaved in an unjustified way, but argues that he is not to blame for his action.
 The notion of justification and excuses will be examined throughout the essay to determine the reasons why a perpetrator either a principal or secondary offender should be afforded to be given the defence. SELF DEFENCE Under common law, it has long been accepted that a citizen is allowed to use reasonable force to protect his own person, his property and the person of another. It is the universally acknowledged notion that no one should yield to unlawful encroachments on his life, bodily integrity or property.
 A similar approach can be found in the statutory provisions of the s3 (1) Criminal Law Act 1967 which permits the use of reasonable force in order to prevent crime or to arrest offenders.  A complete defence will therefore be available when the defendant was facing an unjust threat from the victim and as a result of the threat used a level of force in defence of oneself, another, property or in the prevention of crime provided that the force was necessary and proportionate in the circumstances.
 In Re A (Children), Robert Walker LJ suggested that if a six year old child firing a gun indiscriminately in a school playground was shot and killed by a defendant to prevent further harm the defendant would be able to rely on the defence of self-defence at common law. (a) Rights and forfeiture This approach came to prominence as a result of the work of Suzanne Uniacke who explained the permissibility of killing by self-defence. She explained that a person's right to life or freedom from violence is conditional upon that person not posing a threat to other people.
 Where an aggressor threatens to violate the attacked person's right to life or right to bodily integrity, the attacked person will be justified in using defensive force to repel the threat because she is simply protecting her right to life. In becoming an immediate threat to the life of another, the aggressor, temporarily forfeits her right to life as long as she remains a threat.  (b) Rights of autonomy It is thought that self-defence is justificatory because of the balancing interest involved, that of the defender and that of the aggressor.
Supporters of self-defence were of the view that it is part of one right of autonomy that one can defend himself from an unjust attack.  This justification is based on contemporary theorist idea of core right not to be killed. Wallerstein suggested a different justification based on "forced consequences". He provided that self defence is permitted as necessary response where there is no 'real choice' but to use defensive force. 
Traditionally, it has been suggested that killing to prevent rape is permissible on the basis that it is reasonable to assume that every threat of rape carries with it the threat of death or that rape is equivalent to such a serious injury that, it is therefore proportional to repel the threat with lethal force.  However apart from all the arguments, evidence had held that there is growing evidence that self-defence have become too generous to people who would otherwise bear responsibility for very serious crimes.  NECESSITY
Necessity is a defence which is recognised by the English law only to a limited extent. In Southward v Williams, Lord Denning: "… if homelessness were once admitted as a defence to trespass, no one's house could be safe. Necessity would open a door which no man could shut. " (i) Necessity as a justification The defence of necessity as a justification is also called the 'the lesser of two evils'. Necessity is thought to encompass a general 'balancing of harms' where the D commits a crime to avert an imminent threat to his life or limb, or the life or limb of other persons.
 This can be seen in Re A (Children) where Ward LJ said that, the law must allow the doctors to choose the lesser of the two evils; the lesser evil as regards the case of the twins is to perform the operation. It is argued that since the harm avoided is greater than the harm inflicted, there is no reason why a defence should not be afforded to the perpetrator.  (a) The 'rights' theory The standard account of justificatory necessity is depending on a utilitarian assessment of the competing goods and harms.
However, where two goods are not properly comparable and so a utilitarian approach cannot work, an approach focused on rights will provide a more robust analytical tool.  Gardner was of the opinion that it is much more comfortable to think of necessity in terms of the vindication of rights; that a person would have the defence if he committed what would otherwise be an offence but is actually acted to vindicate a right that is recognise by the law.
 Courts have also been willing to accept a defence of necessity in special defined circumstances where the offence committed is to promote the well-being of a 'victim' who is unable to consent. It was stated in F v West Berkshire Authority that the justification is that patients should be treated in a way which promotes his best interest. Lord Brandon: "… when persons lack the capacity… to take decisions about the performance of operations on them…
it is necessary that some other person or persons, with the appropriate qualifications, should take such decisions for them… " (ii) Necessity as an excuse – Duress of circumstances The defence of necessity as an excuse deals with cases in which an important interest of the actor was about to suffer a serious injury and out of fear and under the pressure of the circumstances, the actor acted in a way that normally constitutes a crime, in order to prevent the injury to himself or to the other person.
If the defendant is able to show that he was 'constrained by circumstances' to act as he did, or that he was 'impelled to act as he did', he should be afforded a defence.  (a) Overwhelming needs In Davidson, the court provided that necessity involves the elements of emergency and proportion. In the circumstances where his will was 'overborne' by the fear, and he did not have a fair opportunity to avoid acting as he did, the rationale of the defence is 'that the actor has been in effect compelled to act as he does by the pressure of a …
threat … it is the impact of that pressure on his freedom to choose his course of action that suffices to excuse him from criminal liability'.  In a German Case referred to by Hitchler, the court in the case held that the act of the accused which in a sudden storm pushed her boyfriend into the water in order to save herself from immediate danger was done in circumstances of emergency, and was therefore not due to her fault. (b) Higher value
Where D's act involves not 'killing one to save one" but killing a smaller number to save a larger number, the principle of "greater value" is appropriate. Smith and Hogan imply that it would be lawful to open a dike, drowning the half-dozen inhabitants of a farm if that were the only way of saving a dam from bursting and inundating the whole town.  In the American case Holmes, Baldwin J: "it is better that some should live than that all perish. " Where a defendant acted under this situation, his should be afforded the defence of necessity and would probably be excused and acquitted.