In VS §12 Pope John Paul II provides a definition of “natural law:”
Only God can answer the question about the good, because he is the Good. But God has already given an answer to this question: he did so by creating man and ordering him with wisdom and love to his final end, through the law which is inscribed in his heart (cf. Rom 2:15), the “natural law”. The latter “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”. [Emphasis in original]
God did not leave us without a means of being able to know how we ought to live; the natural law given to us provides us one. The same is said by St Thomas:
As stated above (90, 1, ad 1), law, being a rule and measure, can be in a person in two ways: in one way, as in him that rules and measures; in another way, as in that which is ruled and measured, since a thing is ruled and measured, in so far as it partakes of the rule or measure. Wherefore, since all things subject to Divine providence are ruled and measured by the eternal law, as was stated above (Article 1); it is evident that all things partake somewhat of the eternal law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends. Now among all others, the rational creature is subject to Divine providence in the most excellent way, in so far as it partakes of a share of providence, by being provident both for itself and for others. Wherefore it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the rational creature is called the natural law. Hence the Psalmist after saying (Psalm 4:6): “Offer up the sacrifice of justice,” as though someone asked what the works of justice are, adds: “Many say, Who showeth us good things?” in answer to which question he says: “The light of Thy countenance, O Lord, is signed upon us”: thus implying that the light of natural reason, whereby we discern what is good and what is evil, which is the function of the natural law, is nothing else than an imprint on us of the Divine light. It is therefore evident that the natural law is nothing else than the rational creature’s participation of the eternal law. [ST I-II q91 a2, emphasis added]
What is the eternal law of which he speaks here?
[A] law is nothing else but a dictate of practical reason emanating from the ruler who governs a perfect community. Now it is evident, granted that the world is ruled by Divine Providence, as was stated in the I, 22, A1,2, that the whole community of the universe is governed by Divine Reason. Wherefore the very Idea of the government of things in God the Ruler of the universe, has the nature of a law. And since the Divine Reason’s conception of things is not subject to time but is eternal, according to Proverbs 8:23, therefore it is that this kind of law must be called eternal. [ST I-II q91 a1]
So natural law is nothing other than the exercise of reason by which we discern what is good and what is evil, something we are empowered by God to do by virtue of being created as rational beings in His image.
But this is not the only way that God makes good and evil, right and wrong, known to us. JPII continues:
He also did so in the history of Israel, particularly in the “ten words”, the commandments of Sinai, whereby he brought into existence the people of the Covenant (cf. Ex 24) and called them to be his “own possession among all peoples”, “a holy nation” (Ex 19:5-6), which would radiate his holiness to all peoples (cf. Wis 18:4; Ez 20:41). The gift of the Decalogue was a promise and sign of the New Covenant, in which the law would be written in a new and definitive way upon the human heart (cf. Jer 31:31-34), replacing the law of sin which had disfigured that heart (cf. Jer 17:1). In those days, “a new heart” would be given, for in it would dwell “a new spirit”, the Spirit of God (cf. Ez 36:24-28). [Emphasis in original]
God also revealed these things to us. Why? St Thomas provides an answer.
It was necessary for man’s salvation that there should be a knowledge revealed by God besides philosophical science built up by human reason. Firstly, indeed, because man is directed to God, as to an end that surpasses the grasp of his reason: “The eye hath not seen, O God, besides Thee, what things Thou hast prepared for them that wait for Thee” (Isaiah 64:4). But the end must first be known by men who are to direct their thoughts and actions to the end. Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation. Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors. Whereas man’s whole salvation, which is in God, depends upon the knowledge of this truth. Therefore, in order that the salvation of men might be brought about more fitly and more surely, it was necessary that they should be taught divine truths by divine revelation. It was therefore necessary that besides philosophical science built up by reason, there should be a sacred science learned through revelation. [ST I q1 a1; emphasis added]
Arriving at truth by means of reason is difficult, and not all have the necessary gifts to discern it reliably. Consequently God in His gracious mercy revealed truth to men so that they could know with certainty those things that they need to know in order to be saved. This fact doesn’t mean that we should not reason about right and wrong. It simply means that we do not have to rely solely upon it in order to live moral lives.