From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
Patterned on God’s freedom, man’s freedom is not negated by his obedience to the divine law; indeed, only through this obedience does it abide in the truth and conform to human dignity. This is clearly stated by the Council: “Human dignity requires man to act through conscious and free choice, as motivated and prompted personally from within, and not through blind internal impulse or merely external pressure. Man achieves such dignity when he frees himself from all subservience to his feelings, and in a free choice of the good, pursues his own end by effectively and assiduously marshalling the appropriate means”.75
In his journey towards God, the One who “alone is good”, man must freely do good and avoid evil. But in order to accomplish this he must be able to distinguish good from evil. And this takes place above all thanks to the light of natural reason, the reflection in man of the splendour of God’s countenance. Thus Saint Thomas, commenting on a verse of Psalm 4, writes: “After saying: Offer right sacrifices (Ps 4:5), as if some had then asked him what right works were, the Psalmist adds: There are many who say: Who will make us see good? And in reply to the question he says: The light of your face, Lord, is signed upon us, thereby implying that the light of natural reason whereby we discern good from evil, which is the function of the natural law, is nothing else but an imprint on us of the divine light”.76 It also becomes clear why this law is called the natural law: it receives this name not because it refers to the nature of irrational beings but because the reason which promulgates it is proper to human nature.77
75 Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 17.
76 Summa Theologiæ, I-II, q. 91, a. 2.
77 Cf. Catechism of the Catholic Church, No. 1955.
— Veritatis Splendor 42