A defense that may be raised to a crime. Essentially, the defendant claims that the suffering of a mental defect or disease prevents the person from understanding and appreciating the wrongfulness of his or her actions or to distinguish right from wrong. For example, those who are deemed criminally insane are unable to be convicted of a crime since the crime involves a conscious intent commit a wrong, which cannot be the case when someone is insane. Also see irresistible impulse and the McNaghten Rule.