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The right of self-defense (also called, when it applies to the defense of another, alter ego defense, defense of others, defense of a third person) is the right for civilians acting on their own behalf to engage in violence for the sake of defending one's own life or the lives of others, including the use of deadly force.
The early theories make no distinction between defense of the person and defense of property. Whether consciously or not, this builds on the Roman Law principle of dominium where any attack on the members of the family or the property it owned was a personal attack on the pater familias – the male head of the household, sole owner of all property belonging to the household, and endowed by law with dominion over all his descendants through the male line no matter their age. In Leviathan (1651), Hobbes proposed the foundation political theory that distinguishes between a state of nature where there is no authority and a modern state. Hobbes argues that although some may be stronger or more intelligent than others in their natural state, none are so strong as to be beyond a fear of violent death, which justifies self-defense as the highest necessity. In the Two Treatises of Government, John Locke asserts the reason why an owner would give up their autonomy:
In earlier times before the development of national policing, an attack on the family home was effectively either an assault on the people actually inside or an indirect assault on their welfare by depriving them of shelter and/or the means of production. This linkage between a personal attack and property weakened as societies developed but the threat of violence remains a key factor. As an aspect of sovereignty, in his 1918 speech Politik als Beruf (Politics as a Vocation), Max Weber defined a state as an authority having the monopoly of the legitimate means of organised violence within defined territorial boundaries (see Weber's Thesis). Recognizing that the modern framework of nations has emerged from the use of force, Weber asserted that the exercise of power through the institutions of government remained indispensable for effective government at any level which necessarily implies that self-help is limited if not excluded.
For modern theorists, the question of self-defense is one of moral authority within the nation to set the limits to obedience to the state and its laws given the pervasive dangers in a world full of weapons. In modern societies, states are increasingly delegating or privatizing their coercive powers to corporate providers of security services either to supplement or replace components within the power hierarchy. The fact that states no longer claim a monopoly to police within their borders, enhances the argument that individuals may exercise a right or privilege to use violence in their own defense. Indeed, modern libertarianism characterizes the majority of laws as intrusive to personal autonomy and, in particular, argues that the right of self-defense from coercion (including violence) is a fundamental human right, and in all cases, with no exceptions, justifies all uses of violence stemming from this right, regardless whether in defense of the person or property. In this context, note that Article 12 Universal Declaration of Human Rights states:
The inclusion of defense of one's family and home recognizes the universal benefit claimed to stem from the family's peaceable possession of private property. This general approach implicitly attacks Hohfeld's focus on the correlative relationship between right and duty as an aspect of human interactiveness as opposed to rights deemed implicitly more important because they attach to a person by virtue of his or her ownership of property. Further, it follows that, in this moral balancing exercise, laws must simultaneously criminalize aggression resulting in loss or injury, but decriminalize qualitatively identical violence causing loss or injury because it is used in self-defense. As a resolution of this apparent paradox and in defiance of Hohfeld, Robert Nozick asserted that there are no positive civil rights, only rights to property and the right of autonomy. In this theory, the "acquisition principle" states that people are entitled to defend and retain all holdings acquired in a just way and the "rectification principle" requires that any violation of the first principle be repaired by returning holdings to their rightful owners as a "one time" redistribution. Hence, in default of self-defense in the first instance, any damage to property must be made good either in kind or by value. Similarly, theorists such as George Fletcher and Robert Schopp have adopted European concepts of autonomy in their liberal theories to justify the right-holder using all necessary force to defend his or her autonomy and rights. This right inverts the felicitation principle of utilitarianism with the responsive violence being the greatest good to the individual, but accurately mirrors Jeremy Bentham who saw property as the driving force to enable individuals to enhance their utilities through stable investment and trade. In liberal theory, therefore, to maximise the utility, there is no need to retreat nor use only proportionate force. The attacker is said to sacrifice legal protection when initiating the attack. In this respect, the criminal law is not the tool of a welfare state which offers a safety net for all when they are injured. Nevertheless, some limits must be recognized as where a minor initial attack simply becomes a pretext for an excessively violent response. The civil law systems have a theory of "abuse of right" to explain denial of justification in such extreme cases.
In most jurisdictions, defense of self or of others is an affirmative defense to criminal charges for an act of violence. It acts to provide complete justification when the degree of violence used is comparable or proportionate to the threat faced, so deadly force would only be excused in situations of "extreme" danger. The defense would fail, for example, if a defendant deliberately killed a petty thief who did not appear to be a physical threat. Likewise, when an assailant ceases to be a threat (e.g. by being tackled and restrained, surrendering, or fleeing), the defense will fail if the defending party presses on to attack. A somewhat less obvious application of this rule is that admitting the use of deadly force in an attempt to disable rather than kill the assailant can be construed as evidence that the defendant wasn't yet in enough danger to justify lethal force in the first place. Sometimes there is a duty to retreat which makes the defense problematic when applied to abusive relationships (see battered woman syndrome and abuse defense), and in burglary situations given the so-called castle exception (see: Edward Coke) which argues that one cannot be expected to retreat from one's own home, namely, "a man's house is his castle, et domus sua cuique est tutissimum refugium" i.e. Latin for "and one's home is the safest refuge"). However, if one is "challenged" in a bar for a fight, accepting such challenge, instead of walking away, generally will not constitute a self defense.
In some countries and U.S. states, the concept of "pre-emptive" self defense is limited by a requirement that the threat be imminent. Thus, lawful "pre-emptive" self defense is simply the act of landing the first-blow in a situation that has reached a point of no hope for de-escalation or escape. Many self-defense instructors and experts believe that if the situation is so clear-cut as to feel certain violence is unavoidable, the defender has a much better chance of surviving by landing the first blow (sucker punch) and gaining the immediate upper hand to quickly stop the risk to their person.
Justification for self-defense usually cannot be applied to actions committed after a criminal act has taken place. A rape victim who, after the rape is committed and the rapist leaves, subsequently finds and shoots the rapist, is not entitled to claim self-defense. Most other victims of assaultive offenses are similarly not entitled to this defense if they act in revenge. In all but one U.S. jurisdiction, using deadly force against a robber or burglar who is attempting to escape with property is likewise not justifiable (Texas is the only exception and holds the defendant to a high burden of proof that the action was the only means available to recover the property without a serious risk of death or serious injury).
The rules are the same when force is used to protect another from danger. Generally, the defendant must have a reasonable belief that the third party is in a position where they would have the right of self defense. For example, a person who unknowingly chances upon two actors practicing a fight would be able to defend their restraint of the one that appeared to be the aggressor. Most courts have ruled that such a defense cannot be used to protect friends or family members who have engaged in an illegal fight. Likewise, one cannot use this to aid a criminal.