A person is not responsible for criminal conduct if at the time of such conduct, as a result of mental disease or defect he lacks substantial capacity either to appreciate the criminality [wrongness] of his conduct or to conform his conduct to the requirements of the law
Adds volition from M'Naghten
Volition v. Cognition
Volition - the ability to conform conduct to legal requirements.
Cognition - the ability to understand the nature and quality of acts
At time of act, party was under defect of reason from disease of mind not to know the nature and quality of the act, or if he did know, did not know it was wrong