From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
At this point the true meaning of the natural law can be understood: it refers to man’s proper and primordial nature, the “nature of the human person”,89 which is the person himself in the unity of soul and body, in the unity of his spiritual and biological inclinations and of all the other specific characteristics necessary for the pursuit of his end. “The natural moral law expresses and lays down the purposes, rights and duties which are based upon the bodily and spiritual nature of the human person. Therefore this law cannot be thought of as simply a set of norms on the biological level; rather it must be defined as the rational order whereby man is called by the Creator to direct and regulate his life and actions and in particular to make use of his own body”.90 To give an example, the origin and the foundation of the duty of absolute respect for human life are to be found in the dignity proper to the person and not simply in the natural inclination to preserve one’s own physical life. Human life, even though it is a fundamental good of man, thus acquires a moral significance in reference to the good of the person, who must always be affirmed for his own sake. While it is always morally illicit to kill an innocent human being, it can be licit, praiseworthy or even imperative to give up one’s own life (cf. Jn 15:13) out of love of neighbour or as a witness to the truth. Only in reference to the human person in his “unified totality”, that is, as “a soul which expresses itself in a body and a body informed by an immortal spirit”,91 can the specifically human meaning of the body be grasped. Indeed, natural inclinations take on moral relevance only insofar as they refer to the human person and his authentic fulfilment, a fulfilment which for that matter can take place always and only in human nature. By rejecting all manipulations of corporeity which alter its human meaning, the Church serves man and shows him the path of true love, the only path on which he can find the true God.
The natural law thus understood does not allow for any division between freedom and nature. Indeed, these two realities are harmoniously bound together, and each is intimately linked to the other.
89 Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 51.
90 Congregation for the Doctrine of the Faith, Instruction on Respect for Human Life in its Origin and on the Dignity of Procreation Donum Vitæ (February 22, 1987), Introduction, 3: Acta Apostolicæ Sedis [AAS] 80 (1988), 74; cf. Paul VI, Encyclical Letter Humanæ Vitæ (July 25, 1968), 10: AAS 60 (1968), 487-488.
91 Apostolic Exhortation Familiaris Consortio (November 22, 1981), 11: AAS 74 (1982), 92.
— Veritatis Splendor 50