(1) A person is not responsible for criminal conduct if at the time of such conduct, as a result of mental illness or intellectual disability, he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of law.
(2) As used in this chapter, the term "mental illness or intellectual disability" does not include an abnormality manifested only by repeated criminal or otherwise antisocial conduct.
(3) A defendant may prove mental illness or intellectual disability, as used in this section, in exculpation of criminal conduct.