3. This new state of things gives rise to new questions. Granted the conditions of life today, and granted the meaning which conjugal relations have with respect to the harmony between husband and wife and to their mutual fidelity, would not a revision of the ethical norms, in force up to now, seem to be advisable, especially when it is considered that they cannot be observed without sacrifices, sometimes heroic sacrifices?

And again: by extending to this field the application of the so-called "principle of totality," could it not be admitted that the intention of a less abundant but more rationalized fecundity might transform a materially sterilizing intervention into a licit and wise control of birth? Could it not be admitted, that is, that the finality of procreation pertains to the ensemble of conjugal life, rather than to its single acts? It is also asked whether, in view of the increased sense of responsibility of modern man, the moment has not come for him to entrust to his reason and his will, rather than to the biological rhythms of his organism, the task of regulating birth. 

Competency of the Magisterium

4. Such questions required from the teaching authority of the Church a new and deeper reflection upon the principles of the moral teaching on marriage: a teaching founded on the natural law, illuminated and enriched by divine revelation.

No believer will wish to deny that the teaching authority of the Church is competent to interpret even the natural moral law. It is, in fact, indisputable, as our predecessors have many times declared, (1) that Jesus Christ, when communicating to Peter and to the apostles His divine authority and sending them to teach all nations His commandments, (2) constituted them as guardians and authentic interpreters of all the moral law, not only, that is, of the law of the Gospel, but also of the natural law, which is also an expression of the will of God, the faithful fulfillment of which is equally necessary for salvation. (3)

Conformably to this mission of hers, the Church has always provided-and even more amply in recent times-a coherent teaching concerning both the nature of marriage and the correct use of conjugal rights and the duties of husband and wife.