The insanity defense has existed since the 12th century and is one of the most controversial but one of the least used criminal defense strategies. It claims that a criminal defendant should not be found guilty because of the defendant's insanity. A person who is insane is said to lack the understanding of the crime or the difference between right and wrong. Initially, the defense could be used only to pardon a criminal's sentence. The concept that one's insanity could avoid the sentence began in the 19th century in Isaac Ray's "The Medical Jurisprudence of Insanity" and the English "M'Naughton Case".