In criminal law, insanity is a defence that arises from a claim that the perpetrator was acting under the influence of an `internal' derrangement of the mind. An `external' derrangement may constitute automatism .Lawton LJ in R v Quick stated that "...this quagmire of law seldom entered nowadays save by those in desperate need of some kind of defence". Further in Woolmington v. DPP it was led the prosecution must prove beyond a reasonable doubt the constituent elements of any crime charged against D and to disprove beyond a reasonable doubt any defence save insanity that D may raise at his trial.
The legal definition of insanity stems from an 1843 case, and has not developed to take account of medical and legal progress since then. As long ago as 1953, medical evidence given to the Royal Commission on Capital Punishment showed that even then the rules were considered by doctors to be based on 'an entirely obsolete and misleading conception of the nature of insanity'
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