1. In the Encyclical "Humanae vitae" we read: "The Church, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches as absolutely required that in any use whatever of marriage there must be no impairment of its natural capacity to procreate human life" (HV 11).
At the same time this same text considers and even puts emphasis on the subjective and psychological dimension when it speaks of the "significance," and precisely of the "two significances of the marital act."
The "significance" becomes known with the rereading of the (ontological) truth of the object. Through this rereading, the (ontological) truth enters, so to speak, into the cognitive dimension: subjective and psychological.
"Humanae vitae" seems to draw our attention especially to this latter dimension. This is confirmed, among other ways, indirectly, also by the following sentence: "We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason" (HV 12).
2. That "reasonable character" concerns not only the truth of the ontological dimension, namely, that which corresponds to the fundamental structure of the marital act, but it concerns also the same truth in the subjective and psychological dimension, that is to say, the correct understanding of the intimate structure of the marital act, that is, the adequate rereading of the significances corresponding to this structure and of their inseparable connection, in view of a morally right behavior. Herein lies regulation of human acts in the sphere of sexuality. In this sense we say that the moral norm is identified with the rereading, in truth, of the "language of the body."
3. The Encyclical "Humanae vitae", therefore, contains the moral norm and its reason, or at least an examination of what constitutes the reason for the norm. Moreover, since in the norm the moral value is expressed in a binding way, it follows that acts in conformity with the norm are morally right, while acts contrary to it are intrinsically illicit. The author of the encyclical stresses that this norm belongs to the "natural law," that is to say, it is in accordance with reason as such. The Church teaches this norm, although it is not formally (that is, literally) expressed in Sacred Scripture, and it does this in the conviction that the interpretation of the precepts of natural law belongs to the competence of the Magisterium.
However, we can say more. Even if the moral law, formulated in this way in the Encyclical "Humanae vitae", is not found literally in Sacred Scripture, nonetheless, from the fact that it is contained in tradition and--as Pope Paul VI writes--has been "very often expounded by the Magisterium" (HV 12) to the faithful, it follows that this norm is in accordance with the sum total of revealed doctrine contained in biblical sources (cf. HV 4).
4. It is a question here not only of the sum total of the moral doctrine contained in Sacred Scripture, of its essential premises and the general character of its content, but of that fuller context to which we have previously dedicated numerous analyses when speaking about the "theology of the body."
Precisely against the background of this full context it becomes evident that the above-mentioned moral norm belongs not only to the natural moral law, but also to the moral order revealed by God: also from this point of view, it could not be different, but solely what is handed down by Tradition and the Magisterium and, in our days, the Encyclical "Humanae vitae" as a modern document of this Magisterium.
Paul VI writes: "We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason" (HV 12). We can add: They are capable also of seeing its profound conformity with all that is transmitted by Tradition stemming from biblical sources. The bases of this conformity are to be sought especially in biblical anthropology. Moreover, we know the significance that anthropology has for ethics, that is, for moral doctrine. It seems to be totally reasonable to look precisely in the "theology of the body" for the foundation of the truth of the norms that concern the fundamental problematic of man as "body": "the two will become one flesh" (Gen 2:24).
5. The norm of the Encyclical "Humanae vitae" concerns all men, insofar as it is a norm of the natural law and is based on conformity with human reason (when, it is understood, human reason is seeking truth). All the more does it concern all believers and members of the Church, since the reasonable character of of fertility according to the doctrine contained in the Encyclical "Humanae vitae" (HV 19), and in the Exhortation "Familiaris consortio". The description of "natural," attributed to the morally correct regulation of fertility (following the natural rhythms, cf. HV 16), is explained by the fact that that manner of conduct corresponds to the truth of the person and therefore to his dignity: a dignity which by "nature" belongs to man as a rational and free being.