Western ethical tradition, as far back as Aristotle and even further, has acknowledged that moral truth exists: that its existence does not depend upon the customs or opinions of individuals or groups; that people can know it exists and so know the difference between right and wrong; that its knowability is, as it were, part of the order of the universe; and that all people are bound to obedience to this natural moral order, even if their obedience causes them rejection, suffering and even death.
This naturally knowable law of right and wrong has traditionally been referred to as the "natural law."
Although there are differences between the natural law and law in the ordinary sense of the term ("man - made law"), there are several aspects they share, and indeed that must be shared by anything that is rightly called law. Like man-made law, the natural law directs humans in their deliberations and choices; it secures and promotes individual and communal wellbeing; it is instituted by a valid authority (God); and it is knowable by all who are bound by it. Unlike other laws, however, it is not written down in the first place in statute books or on tablets of stone. Rather, as St. Paul says, it is written "on our hearts" (see Rom. 2:15).
This Pauline expression "written on our hearts" means that the natural law is something that corresponds to our very nature, and that we discover it by using our natural reasoning capacities. When we think about what to do, about what is good and choiceworthy and what we find fulfilling, we see - understand - that there are certain goods that all human beings are interested in, goods that correspond to the fulfillment of the very capacities of our natures, goods such as health, friendship, marriage and family, knowledge, skillful performance, aesthetic experience and inner peace. Nobody has to tell us that these things are good, that, for example, life and health are preferable to death and disease, or that having a firm intellectual grasp on reality is better and more choiceworthy than living in ignorance and muddle; we know these things "naturally."
We then go on to think about ways to realize these more basic goods for ourselves and those around us, and in so doing we generate options for choice. These options - such as getting married to this woman, or robbing that bank - are all ultimately related to our plans to realize the more basic goods of life, friendship, etc. The basic goods are the ends we are trying to bring about, and the plans are the means we think might realize those ends. As we deliberate over these means-to-ends we come to see that some are compatible with the wider good of our selves, neighbors and communities, and others, though perhaps efficacious for realizing the ends we seek, entail harming other goods. For example, in thinking about ways to support my family, I might rightly conclude that robbing a bank is an efficacious way to generate a lot of loot in a short time. But I also see it would involve hurting other people's financial situations and maybe even shooting a few people in the process. So through this kind of deliberation (or moral reasoning) we come to affirm that certain ways of realizing the goods we seek are morally good - i.e., consistent with the well being of those upon whom our actions bear - and others are not. And we codify this knowledge in formulations such as 'gainful employment is good and should be pursued' and 'theft and murder are bad and should be avoided.'
When we say the natural law is naturally knowable we mean the human mind can grasp its content without the aid of divine revelation. Now divine revelation offers great assistance to people of faith in helping them to know how to live. But if morality were only knowable through divine revelation, then non-believers would be cut off from moral truth. But defenders of the natural law argue that the moral truths taught in divine revelation such as the Ten Commandments are knowable quite independently of the revelation to Moses on Mt. Sinai. They argue that Christians and non-Christians alike can know that murder and adultery and covetousness and lying are wrong and should not be done.
The natural law just is the objective moral law created by God and promulgated in creation. It contains truths that unaided reason can grasp and that direct human action towards those ends and means that protect and promote the goods of our nature. The tradition has referred to it as a kind of intellectual "light" implanted in us by God enabling us to see what is right and good and so making possible for all a life of good works consistent with human flourishing. In a community where that light seems to be going out, it is all the more important for Christians and all defenders of an objective moral law confidently, and whenever possible publically, to defend the truths of the natural law, even while they themselves may need to rest their minds in that certitude of the moral truth that Christian divine revelation guarantees.