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Self defence, if successfully pleaded, is a a defence to a criminal charge or a claim of Trespass to the person. The common-law has always recognized that there are circumstances in which one may use violence to protect oneself. Self-defence overlaps with the statutory provisions regarding prevention of crime -- in many cases a person employing self-defence is also preventing a crime.
Difficulties arise in two well-known scenarios. The first is where the defence used is disproportionate to the threat. It clearly is not acceptable to prevent someone slapping you round the face by cutting the offending arm off. On the other hand, if your own life is clearly and imminently threatened there are few limits on what you may do in self defence. Between these two extremes are any number of difficult cases.
The second scenario is where the defendant has misjudged the severity of the threat. It appears that in the criminal law what is important is the defendant's own understanding of the situation. If you genuinely and honestly believe that I am about to hit you on the head and run off with your wallet, even if I am clearly only combing my hair, then you have a good defence in self defence. In tort, the situation is not so clear. There is very little case-law, but the weight of academic opinion is that for self defence to succeed, your actions must be objectively, not subjectively, reasonable.
In both crime and tort there are special problems when the person using self defence is intoxicated. It is very likely the case that a drunkard is unable to determine either the severity of a thread, or the degree of force needed to avert it. However, for all the usual reasons that courts have been reluctant to allow intoxication as a reason why self defence might have been misdirected.