The provision of defence counsel in criminal trials from 1836 sparked an unforeseen debate on how to establish a reliable and consistent means for determining a prisoner's mental state in questions of criminal responsibility. The landmark tort trial of Vaughan v. Menlove (1837) established the ‘reasonable man’ standard by which juries should base their decisions on the grounds that ‘whether the Defendant had acted honestly and bona fide to the best of his own judgment … would leave so vague a line as to afford no rule at all … we ought rather to adhere to the rule which requires in all cases a regard to caution such as a man of ordinary prudence would observe’. While this seemed to offer a transferable standard with which to judge cases of legal culpability when dealing with a ‘reasonable man’, it could not be applied in cases of insanity where reason was the very quality of mind under consideration. The most notorious homicide case of the century raised the question of the definition of legal insanity. This was the case of Daniel McNaughten who killed Edmund Drummond in 1843 believing him to be Sir Robert Peel, under the delusion that the Tories were his enemies. Rather than leaving the jury to use the ‘reasonable man’ standard, which he had established in Vaughan v. Menlove, in McNaughten's trial, Justice Tindal directed the jury to acquit. Without retiring to deliberate, the jury returned a verdict of not guilty on the ground of insanity.
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