393.063(12), Florida Statutes. The idea behind the insanity defense is that a criminal defendant who is insane lacks the intent necessary to perform a criminal act.Florida uses the M'Naghten Rule for the insanity defense, says Broward criminal attorney Moore. The criminal defendant is presumed to be sane. Although widely popularized in the media as a way for defendants to avoid repercussions, few criminal defendants plead insanity. In an insanity defense, the defendant admits the action but asserts a lack of culpability based on mental illness. The idea of insanity may be outdated, but it still possible for a defendant's mental illness to influence the outcome of a criminal case.