A person is not criminally responsible for conduct if at the time of such conduct he was laboring under such a defect of reason, from disease of the mind as not to know the nature and quality of the act he was doing, or if he did know it, that he did not know what he was doing was wrong. Insanity is an affirmative defense which must be proved by a preponderance of the evidence.
N.J.S. § 2C:4-1