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The standard rules applied to the determination of whether a person who offers an Insanity defence was insane at the time of the commission of the offence. The Rules derive from a debate in the House of lords concerning the outcome of Mnaghtens case (1843). The relevant passage is ...it must be clearly proved that, at the time of committing the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing, or, if he did know it, that he did not know he was doing what was wrong.
Analyzed in detail, this statement says that the accused must be suffering from a 'disease of the mind', and the disease gave rise to a 'defect of reason', and he was not aware of the nature of the act did not know he was doing wrong.
Disease of the mind A 'disease of the mind' need not be a disease of the brain, or even a psychiatric condition: arteriosclerosis, epilepsy and diabetes have all be held to be forms of insanity. The rationale is that all have an effect on the 'mind', and all result from internal factors. On the contrary, a blow to the head resulting in mental derangement will probably lead to acts classifiable as Automatism rather than insanity. Although there is no logical or medical basis for this, it may have the practical benefit that 'internal' factors are liable to recur, and therefore require detention for treatment.
Defect of reason The disease of the mind must interfere with the process of reasoning or understanding. The defence does not exist for a person who simply fails to think about his actions, perhaps as a result of depression.
Nature of the act The accused must show that he was unaware of the nature of the act, not the moral or legal nature. A person who believes he was not doing wrong may, however, escape liability under the other limb of the M'Naghten test (as described below). A person who kills someone under the delusion that he is on a battlefield may escape liability in this way. A person who is so deluded that he stabs someone thinking he is cutting down a tree may also escape, but he also lacks the Mens Rea of murder, so may not be liable anyway.
Whether the accused knew he was doing 'wrong' If the accused knew that the act was illegal, then he knew he was 'doing wrong' for the purpose of this test. In other words, legal wrongness, not moral wrongness, is important.
Note that an irresistable impulse to commit a crime cannot found a defence of insanity, however strong.