Dr. Cannon's Mere Marriage proposes that Saint John Paul heralded a development of doctrine by his teaching on the theological meaning of sexual difference. In the words of John Paul's friend and mentor Henri de Lubac S. J.: "When something so fundamental] is called into question you have to get down to its roots....to meet the strategic demands of the situation, ...the whole matter has to be studied from a new viewpoint. It is not so much a question of justifying as elucidating; polemic is not the primary concern. Fundamentally, what you feel is a need to see straight; it is a time for stocktaking." To show how Saint John Paul followed de Lubac's counsel, Cannon delves into Church Fathers, as well as Medieval and Modern theologians, to reacquaint readers with the anthropological foundations of sacred tradition. He contrasts natural law's ethical reasoning which prioritizes objective principles, with personalism's prioritizing of concrete individuals. He contrasts personalistic ontologies with the metaphysics of being showing that each uses different approaches and achieves different kinds of inferences. Against this background, Cannon compares Humanae Vitae and the Theology of the Body, respectively, as naturalistic and the personalistic treatises, on conjugal love. He shows how the latter's personalistic inferences develop the former's naturalistic inferences. He then scrutinizes John Paul's radical inferences as a potential development of doctrine according to Saint John Henry Newman's guidance, and concludes that John Paul's proposal cannot be disqualified as a potential development of doctrine according to Newman's seven tests.