Today is the feast of Pope St. John Paul II. From Veritatis splendor:
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".
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.