What do Catholics mean when they appeal to the “natural law”?


What exactly is this natural law? What is its content and how do we come to know it? And is it possible to convince someone with whom one disagrees of its truth?

In public arguments regarding abortion, euthanasia, homosexual marriage, and other politically combustible issues, one often hears defenders of the Catholic position appeal to something called the “natural law.” The point of the appeal seems to be that there are certain fundamental moral truths, certain natural laws, that can be acknowledged by Catholics, non-Catholics, and even non-believers. Arguments based upon natural law are meant to cut through differences in religious and philosophical belief to some incontestable common ground and thus to provide a basis for moral consensus. What exactly is this natural law? What is its content and how do we come to know it? And is it possible to convince someone with whom one disagrees of its truth?   
The natural law, as philosopher Robert Sokolowski defines it, is “the ontological priority of ends over purposes.” What does this rather intimidating phrase mean? First, ends, which belong to things according to their natures, enjoy an ontological priority, a priority in being, to the purposes (i.e., the particular designs) that human beings intend. Ends, in other words, exist by natural necessity, purposes by the contingency of human choice. “Natural law,” Sokolowski says, “is shown to us when we recognize that there are ends in things and that our purposes and choices must respect their priority.”
Consider, for example, a young man who decides to take a class in Ancient Greek simply because he wants to impress one of the young ladies in the class. The young man’s purpose, his motive, is to impress the girl. But this is not the end of the study of Ancient Greek. The end, or point, of studying Ancient Greek is to master the language sufficiently so that one can read and enjoy texts written in that language. The reading and enjoyment of ancient texts is “built in” to the study of Ancient Greek as its end, and so is not a matter of choice for those who would enter the Ancient Greek classroom, whatever their individual purposes may be. The end of studying Ancient Greek enjoys an ontological priority, a priority in being, to the purposes of students.
Consider now a more morally serious example. One of the ends of sexual intercourse is the begetting of children. Children, if we can put it this way, are part of what “nature intends” by this act. Human beings, however, in the grip of their purposes, have discovered ingenious ways in which to indulge in the pleasures of sexual intercourse without involving themselves in the heavy responsibility of children. Contraception, in other words, is a purpose that disrupts the natural end of sexual intercourse. Contraception is thus against the natural law. It undermines the priority that ends have over purposes. 
We are seeing that the notion of natural law presupposes an understanding of human nature with ends built into it. We human beings are made for certain types of activity. Most fundamentally, we are made to maintain ourselves in existence. But we are also made to live in families, to beget children, to cultivate friendship, to form political communities, and to pursue the truth – especially the truth about God. All of these ends are essential to the flourishing of human nature, even though some ends (such as knowing the truth about God) are more important than others (maintaining ourselves in existence). 
Who is responsible for building these ends into human nature? God, of course. God created our nature and ordered it to a hierarchy of ends culminating in himself. But God did more than this – he also put our natural inclinations to ends under law. 
In order to understand what God’s legal instruction involves, we first need to understand what law itself is. St. Thomas Aquinas, who in his Summa theologiæ wrote the most famous discussion of law in the Catholic tradition, defines law as “a certain ordination of reason to the common good, made by him who has care for the community, and promulgated.” Law, by this definition, is not a sheer act of power, but is essentially rational. It is a work of reason made and promulgated by political authority for the sake of the common good. Divine providence – i.e. the ordering of everything in the cosmos to God – is just such a work of reason, and so Aquinas refers to divine providence as a kind of law: specifically, the eternal law. God himself is both the political authority establishing the eternal law and the common good to which the eternal law is ordered. The natural law is a kind of subset of the eternal law; it is the eternal law as manifested in rational creatures, or as Aquinas defines it, the participation of the rational creature in the eternal law.
These distinctions help us see more clearly the ontological priority of ends over purposes that Sokolowski defines as natural law. Because God’s own reason is the source of natural law, the commands he gives to our rational natures are prior in being to the purposes we set up for ourselves. But Aquinas’s definitions of law and natural law also help us see that the natural law is not, in the most precise sense, the ends themselves of human nature. Rather, the natural law is the rule of reason that governs the appropriate pursuit of our ends. Law or precept, Aquinas says, implies order to an end, insofar as that which is commanded is either necessary or expedient for the attainment of an end.
So how do we come to know the natural law? The very first precepts of the natural law are self-evident to the human mind. For example, as soon as we have even a vague comprehension that we are human beings and that we are alive, we also and immediately comprehend that our existence must be maintained (we cannot logically go circumvent this conclusion). There is no need for proof of any kind, understanding proof as a series of logical steps leading from a set of premises to a conclusion. No such discursive movement of the mind is necessary when it comes to grasping the first precepts of the natural law. They are not demonstrated so much as they are “seen” by the intellect. 
While in principle all human beings, no matter their religious or philosophical beliefs, share some common moral ground in the first precepts of the natural law, this does not mean that the natural law all by itself has the power to resolve all our moral differences. This is in part because natural law involves more than the self-evident first precepts. The natural law also involves secondary precepts derived from the first precepts, and in some way, too, all of those nuanced judgments of prudence that are based on the first and secondary precepts. The more and more specific our moral decisions become, the more room there is for error.   
But another factor preventing natural law from being recognized by all is the ill-formed moral habits that prevent us from recognizing the priority of ends over purposes. While, for example, the natural law precept commanding us to maintain ourselves in existence is self-evident, the person contemplating suicide – perhaps out of a despair that is no fault of his own – is in the grip of a purpose, suicide, that clouds over the priority of life. In similar fashion, the procreative end of sexual intercourse is ignored by those who practice contraception and abortion. Indeed, the procreative end of sex is regarded by many as just one more human purpose – a purpose they do not happen to share.
Here is the rub of our cultural problem: In embracing an ever more militant secularism, our culture has jettisoned both the conception of nature upon which natural law depends and the God to whom nature owes its being and to whom it is directed. Nature in our pseudo-scientific outlook has no ends; it is only an arena of competing forces. This outlook holds for human nature as well as for non-human nature. Human beings, on many people’s viewpoint, have no ends to which they are ordered. They are under no natural law pointing out the way to the good. Human beings only have their purposes, and morality is nothing other than a sociological or evolutionary phenomenon: a recognition of shared purpose that allows us, more or less, to get along peacefully.     
The recovery of natural law requires, therefore, the recovery of a proper understanding of nature. We cannot hope to forge a moral consensus in our secular, pluralist society if we cannot appeal to the distinction between ends and purposes. But even more importantly than this, a genuine moral consensus rooted in natural law requires, paradoxically, renewed devotion to the Triune God. Grace, as St. Thomas Aquinas teaches us, builds upon nature. The entire point of natural law is to lead us to God. But nature cannot take us all the way. In order to enjoy a supernatural life with God we need supernatural power to help us get there. But in helping us get to God, grace does not destroy nature but rather completes it, and in completing nature the light of grace is able to show us more perfectly than can our natural lights, especially in our fallen state, what the natural law is and what it’s for. 
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