From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
The Church has often made reference to the Thomistic doctrine of natural law, including it in her own teaching on morality. Thus my Venerable Predecessor Leo XIII emphasized the essential subordination of reason and human law to the Wisdom of God and to his law. After stating that “the natural law is written and engraved in the heart of each and every man, since it is none other than human reason itself which commands us to do good and counsels us not to sin”, Leo XIII appealed to the “higher reason” of the divine Lawgiver: “But this prescription of human reason could not have the force of law unless it were the voice and the interpreter of some higher reason to which our spirit and our freedom must be subject”. Indeed, the force of law consists in its authority to impose duties, to confer rights and to sanction certain behaviour: “Now all of this, clearly, could not exist in man if, as his own supreme legislator, he gave himself the rule of his own actions”. And he concluded: “It follows that the natural law is itself the eternal law, implanted in beings endowed with reason, and inclining them towards their right action and end; it is none other than the eternal reason of the Creator and Ruler of the universe”.83
Man is able to recognize good and evil thanks to that discernment of good from evil which he himself carries out by his reason, in particular by his reason enlightened by Divine Revelation and by faith, through the law which God gave to the Chosen People, beginning with the commandments on Sinai. Israel was called to accept and to live out God’s law as a particular gift and sign of its election and of the divine Covenant, and also as a pledge of God’s blessing. Thus Moses could address the children of Israel and ask them: “What great nation is that that has a god so near to it as the Lord our God is to us, whenever we call upon him? And what great nation is there that has statutes and ordinances so righteous as all this law which I set before you this day?” (Dt 4:7-8). In the Psalms we encounter the sentiments of praise, gratitude and veneration which the Chosen People is called to show towards God’s law, together with an exhortation to know it, ponder it and translate it into life. “Blessed is the man who walks not in the counsel of the wicked, nor stands in the way of sinners, nor sits in the seat of scoffers, but his delight is in the law of the Lord and on his law he meditates day and night” (Ps 1:1-2). “The law of the Lord is perfect, reviving the soul; the testimony of the Lord is sure, making wise the simple; the precepts of the Lord are right, rejoicing the heart; the commandment of the Lord is pure, enlightening the eyes” (Ps 18(19):8-9).
83 Encyclical Letter Libertas Præstantissimum (June 20,1888): Leonis XIII P.M. Acta, VIII, Romæ 1889, 219.
— Veritatis Splendor 44