From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
The teaching of the [Second Vatican] Council emphasizes, on the one hand, the role of human reason in discovering and applying the moral law: the moral life calls for that creativity and originality typical of the person, the source and cause of his own deliberate acts. On the other hand, reason draws its own truth and authority from the eternal law, which is none other than divine wisdom itself.69 At the heart of the moral life we thus find the principle of a “rightful autonomy”70 of man, the personal subject of his actions. The moral law has its origin in God and always finds its source in him: at the same time, by virtue of natural reason, which derives from divine wisdom, it is a properly human law. Indeed, as we have seen, the natural law “is nothing other than the light of understanding infused in us by God, whereby we understand what must be done and what must be avoided. God gave this light and this law to man at creation”.71 The rightful autonomy of the practical reason means that man possesses in himself his own law, received from the Creator. Nevertheless, the autonomy of reason cannot mean that reason itself creates values and moral norms.72 Were this autonomy to imply a denial of the participation of the practical reason in the wisdom of the divine Creator and Lawgiver, or were it to suggest a freedom which creates moral norms, on the basis of historical contingencies or the diversity of societies and cultures, this sort of alleged autonomy would contradict the Church’s teaching on the truth about man.73 It would be the death of true freedom: “But of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall die” (Gen 2:17).
69 Cf. Saint Thomas Aquinas, Summa Theologiae, I-II, q. 93, a. 3, ad 2um, cited by John XXIII, Encyclical Letter Pacem in Terris (April 11, 1963): Acta Apostolicæ Sedis 55 (1963), 271.
70 Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 41.
71 Saint Thomas Aquinas, In Duo Praecepta Caritatis et in Decem Legis Praecepta. Prologus: Opuscula Theologica, II, No. 1129, Ed. Taurinen (1954), 245.
72 Cf. Address to a Group of Bishops from the United States on the occasion of their ad Limina Visit (October 15, 1988), 6:Insegnamenti, XI, 3 (1988), 1228.
73 Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 47.
— Veritatis Splendor 40