The Catholic Church has no such enthusiasm regarding the “natural law”. Pius XI writes, concerning marriage,
“They are greatly deceived who having underestimated or neglected these means which rise above nature, think that they can induce men by the use and discovery of the natural sciences, such as those of biology, the science of heredity, and the like, to curb their carnal desires. We do not say this in order to belittle those natural means which are not dishonest; for God is the Author of nature as well as of grace, and He has disposed the good things of both orders for the beneficial use of men. The faithful, therefore, can and ought to be assisted also by natural means. But they are mistaken who think that these means are able to establish chastity in the nuptial union, or that they are more effective than supernatural grace” (Pius XI, Casti connubii, art. 101).
In short, you will not succeed in producing virtue in this messed up man unless you give him the supernatural means!
Before Pius XI, Pope Leo XIII excoriated those who think that man can build a successful life upon the precepts of the natural law. In our state – fallen because of Adam’s sin – we cannot live successfully the precepts of natural law without grace; indeed, we cannot even get the details of the natural law correct and without error without the help of divine revelation. Most importantly, our final end is not a natural end. It is a supernatural end, the very Vision and Love of God Unveiled. A lengthy citation from Pope Leo is in order, from his Tametsi futura prospicientibus, arts. 11-12:
“The whole object of Christian doctrine and morality is that “we being dead to sin, should live to justice” (1 Peter ii., 24)-that is, to virtue and holiness. In this consists the moral life, with the certain hope of a happy eternity. This justice, in order to be advantageous to salvation, is nourished by Christian faith…. A system of morality based exclusively on human reason robs man of his highest dignity and lowers him from the supernatural to the merely natural life. Not but that man is able by the right use of reason to know and to obey certain principles of the natural law. But though he should know them all and keep them inviolate through life-and even this is impossible without the aid of the grace of our Redeemer-still it is vain for anyone without faith to promise himself eternal salvation…. We are told that society is quite able to help itself; that it can flourish without the assistance of Christianity, and attain its end by its own unaided efforts. Public administrators prefer a purely secular system of government. All traces of the religion of our forefathers are daily disappearing from political life and administration. What blindness!…
12. So great is this struggle of the passions and so serious the dangers involved, that we must either anticipate ultimate ruin or seek for an efficient remedy. It is of course both right and necessary to punish malefactors, to educate the masses, and by legislation to prevent crime in every possible way: but all this is by no means sufficient. The salvation of the nations must be looked for higher….The common welfare, then, urgently demands a return to Him from whom we should never have gone astray; to Him who is the Way, the Truth, and the Life,-and this on the part not only of individuals but of society as a whole. We must restore Christ to this His own rightful possession. All elements of the national life must be made to drink in the Life which proceedeth from Him: legislation, political institutions, education, marriage and family life, capital and labour.”
Thus, a “purely philosophical” conversation with someone could not succeed in mapping without error the terrain of the precepts of natural law. Recourse to divine revelation is necessary. It will direct me, the friend of the messed up man. Further, my friend, for his own good, needs to accept this map as quickly as possible.