One of the great losses to Western Culture is the increasing refusal to accept that there is a Natural Law to which we may commonly refer. This is especially problematic in pluralistic and secularist societies like the post-Christian West where reference to the sacred text of Scripture is not considered authoritative by many.
Hence, it has been the long practice of the Church, even before secularizing trends to base her witness to the truth not only on Scripture but also on Natural Law. The recourse to such a basis for discussion is now largely impossible for us, as most secularists have adopted a radical skepticism that our nature, and that the reality all around us, has anything to say to us in terms of the moral life. Thus, little discussion is possible between believers and secularists and the impasse is clearly on display in the comboxes of blogs such as this and others.
What is the Natural Law? According to St. Thomas, the natural law is “nothing else than the rational creature’s participation in the eternal law” (I-II.94). There are two reason we call this law “natural.” First, because it is set forth in our very nature itself, and second, because it is manifested to us by the purely natural medium of reason, rather than by supernatural revelation. The law, however, we observe does not rest on some particular element or aspect of our nature (e.g. only the physical). The standard is our whole human nature and also the special ends to which we are directed: e.g. justice, truth, rationality, and openness to the eternal.
For example, in observing our overall nature we rightly conclude, by the use of reason, that it is wrong to indulge the satisfaction of some lower need or tendency in a way that is not properly subordinated to the higher goods. We rightly conclude that reason should maintain a proper order and balance among our conflicting tendencies and desires.
- - Thus, to nourish our bodies is right; but to indulge our appetite for food to the detriment of our overall bodily health or spiritual life is wrong.
- - Self-preservation is right and good, but there are times to accept dangerous and even deadly undertakings when the well-being of wider society requires it.
- - A glass of wine may be good and relaxing, but it is wrong to drink to intoxication, for it is injurious to health, and deprives one of the use of reason, the guide and dictator of conduct.
- – Theft is wrong, because it subverts the basis of social life and sows fear and distrust; so does lying; and man’s nature requires for its proper development that he live in a state of well functioning society.
- - Sexual pleasure is good, but promiscuity of any sort undermines the family, spreads disease and endangers children in innumerable ways from abortion to being raised in less than the ideal setting of a committed, complimentary, and stable marriage of a mature mother and father. Outside of this ideal setting for children, a host of social ills follow as we well know today.
Note that in these examples, we have not referenced Scripture, which is supernatural revelation. Natural Law however is accessed through the unaided operation of reason. Founded in our nature and revealed to us by our reason, the natural law is known to us in the measure that reason brings a knowledge of it home to our understanding. The supreme and primary principles (e.g. not to steal, lie, commit adultery, murder) are known to every one having the actual use of reason and are held in every culture. Another class of conclusions or principles are those which are reached only by a more or less complex course of reasoning. This would be due to the more complex nature of the situation encountered. 
Thus, in effect, Natural Law is the law available to us by the use of natural reason. It presupposes that the existing world is intelligible, that it manifests order, and tends toward a purpose or goal (e.g. sustaining life). It presupposes that the natural world is steeped with meaning, and maintains a vigorous optimism that we, who are rational creatures, can learn from what the natural world and our own human nature testify to us.
But this optimism that creation shouts meaning and truth has suffered many serious blows in Western Culture, in the wake of the radical doubt and skepticism set in motion by the Cartesian revolution of the late 16th and early 17th Centuries. Increasingly, many influential Western philosophers came to articulate that things are ultimately, meaningless. Many scientists have taken up the notion that all the intricate order we can observe is only the result of random chance mutations and that the existing world ultimately has no real or ultimate meaning; it is just a chance accident. Materialists refuse to accept anything beyond physical matter, and reject metaphysical concepts such as justice, love, beauty, longing, and moral sense as mere emanations of brain synapses ultimately signifying nothing. Nihilism and other reductionists tendencies have plagued the West and robbed us, collectively speaking of the optimism that we, our lives, or the existing world have meaning and something to teach us.
Thus it is we who believe who are left holding the candle and who optimistically assert that the existing world is steeped with meaning, with teachings, with intelligibility. From the Christian point of view, God made all things through his “Word” (who is our Lord Jesus Christ). The Greek word Logos points to a kind of “logic” that permeates all things and is discoverable to our human reason. The universe was “thought into being” and thus we who possess reason are able to observe, to recognize, the Law, the reason, and the wisdom that underlies and permeates all things.
So, along with the supernatural Book of Sacred Scripture we also have the natural Book of Creation. The Church esteems them both as pointing to the one truth. Thus there can be no absolute or ultimate conflict between true science and faith. As Catholics, we are frequently considered together with our Fundamentalist and Evangelical brethren who do not often esteem the Book of Creation and Natural Law as we do. There are important distinctions that Catholics uniquely make that are often lost on atheists and secularists. We do not insist that our moral teachings and most of our doctrinal teachings are only available by Scripture, we also strive to show them and demonstrate them by way of natural law and that they are quite often accessible to reason.
Again we may note with sadness that this avenue is of late shutting down. Note because we have changed or moved, but because the world has become doubtful and cynical that the existing world or our bodies have anything to tell us.
One cannot judge individual hearts to be sure, but it is not without sobriety to suggest that some, if not many, who have rejected Natural Law have done so, not out struggle or doubt, but because the existence of any law above them is inconvenient to the moral life they wish to lead. Such judgements may be beyond us in individual cases, but collectively it seems clear that the wholesale abandonment of Natural Law has coincided with the declining West’s collective decision to take a moral holiday.
Perhaps as a prosaic conclusion to the Church’s optimism that the created world shouts forth meaning and truth we can end with the words of St Athanasius. Certainly he writes from the standpoint of faith and his words would matter little to a secularist or atheist. But to we who still have that “old time religion” it is a good reflection on how creation mystically manifests the immanence and wisdom of God.
By his own wisdom and Word, who is our Lord and Savior Christ, the all-holy Father (whose excellence far exceeds that of any creature), like a skillful steersman guides to safety all creation, regulating and keeping it in being, as he judges right. It is right that creation should exist as he has made it and as we see it happening, because this is his will, which no one would deny. For if the movement of the universe were irrational, and the world rolled on in random fashion, one would be justified in disbelieving what we say. But if the world is founded on reason, wisdom and science, and is filled with orderly beauty, then it must owe its origin and order to none other than the Word of God.
He is God, the living and creative God of the universe, the Word of the good God, who is God in his own right…. the Word that created this whole world and enlightens it by his loving wisdom….produced the order in all creation….and gives order, direction and unity to creation.
By his eternal Word the Father created all things and implanted a nature in his creatures. He…in his goodness he governs and sustains the whole of nature by his Word (who is himself also God), so that under the guidance, providence and ordering of that Word, the whole of nature might remain stable and coherent in his light. From a Discourse Against the Pagans by Saint Athanasius, bishop (Nn. 40-42: PG 25, 79-83)