Man participates in the wisdom and goodness of the Creator who gives him mastery over his acts and the ability to govern himself with a view to the true and the good.
The natural law expresses the original moral sense which enables man to discern by reason the good and the evil, the truth and the lie:
The natural law is written and engraved in the soul of each and every man, because it is human reason ordaining him to do good and forbidding him to sin . . . But this command of human reason would not have the force of law if it were not the voice and interpreter of a higher reason to which our spirit and our freedom must be submitted.
Source: Catechism of the Catholic Church (Vatican.va)