While Henri de Lubac was a student of theology he decided to follow up on a suggestion made by P. J. Huby and study the relationship between natural and supernatural in Christian thought. Taking the philosophy of Action by M. Blondel (1893) as his starting point, he showed the difference between the thomist doctrine and the «system» of «pure nature» which had developed since the sixteenth century. While this system was useful for guarding the gratuity of revelation, it was foreign to «Christian philosophy», to which Vatican II referred. To be understood, this Christian philosophy must unite philosophy, theology, and mysticism.
