IV. Natural Law and Human Law
Natural law, again, consists of fundamental principles of practical reason, without which practical reasoning is not possible, insofar as these principles can be regarded as having the force of law, or obligatory force; and these principles do, in fact, have the force of law insofar as they concern the common good of human beings as rational animals. It is a common misconception that 'natural law' is called 'natural' because it gets its law from 'nature', either external nature or internal biology; but this is manifestly false, historically and etymologically: it is called 'natural law' because it is the law natural to us as rational animals. Natural law theory is not a theory of biology, nor is it properly a theory of metaphysics; it is not, in other words, a theory of nature or natures, but an account of practical reason, which is appropriate to human nature. This is absolutely essential to understanding how natural law relates to human-made law.
Natural law itself requires the development of human law, precisely because it is the foundation of practical reasoning. The fundamental precepts of natural law, such as good is to be sought and done, and bad avoided, which is the practical equivalent of the principle of noncontradiction, are universal in their scope. But it is also quite clear that they are general and consistent with any number of possible societies and ways of life, and it would be unreasonable to try to argue that it covers every aspect of them, even when some other common good (such as the common good of the political society or dominion or community of nations in which we live) cannot be had without additional requirements. Sometimes we can get specific requirements by reasoning things out carefully and precisely enough, but this is very difficult, and practical reason often requires, by its very concern with the practical, a degree of approximation that makes it impossible to be this precise. When we added empirically discovered facts to these general principles, we can sometimes also get specific requirements, but these are only as discoverable as the facts they presuppose.
Moreover, when we look at the precepts of natural law, we find that some of the precepts are not primarily concerned with the individual. For instance, children and productive labor are two very obvious examples that involve many precepts that are about what human beings as a society should do, rather than anything specific for the individual human being to do. The precepts that concern what we should do with children -- i.e., that deal with issues like how we should treat sexual partnership insofar as it makes children possible, how we should run society so as to make it good for raising children, or how we should handle things like inheritances -- are in general concerned with human beings as a society. It is a precept of natural law that the human race propagate itself: human beings should have children so that there will be human beings in the next generation. But no given person has a particular obligation to breed. The force of the precept is that we should take generation seriously and that if it were ever the case that the human race could go extinct, we should as a society look for reasonable means to remedy this situation, where 'reasonable means' are means that do not violate some other precept of natural law. Likewise, it is a precept of natural law that human beings should engage in sufficient productive labor to support themselves, in terms of food and resources. But as Aquinas argues at quite considerable length, this is not a precept that everyone should do the manual labor required to feed themselves. It is reasonable, and consistent with the precept, if some people do not do this labor if they provide some real benefit in exchange. That is, it's perfectly fine for only some people to be doing the manual labor required to feed, clothe, etc., the entire population, as long as everyone gets something out of it. Aquinas's example is teachers: teaching is an extraordinarily great social good, so it makes sense for there to be a section of the population that instead of spending all its time feeding itself, devotes its time and effort to good teaching in exchange for food or money for it. The thrust of the precept is that productive labor should be treated seriously and with respect, and that we should as a society make sure that it is done. While other precepts, like those concerned with justice in exchange, will affect what means are acceptable in different circumstances, this precept itself is really just a precept for human beings as a society, and not a rigid requirement for every individual. Other examples could be given from these and other areas; the point is that large parts of natural law are practical reasoning about human society, and thus do not dictate what the actions of an individual should be.
In many such cases, liberty will be in possession, and it will make sense just to leave it up to the individual. But there will be plenty of cases where this will make no sense. Under certain conditions, it is irrational to have no traffic laws; but traffic laws will tend to require specific decisions in matters where it doesn't matter what the decision is, as long as some decision is made. Natural law does not say whether we should drive on the right side or the left side, or whether we should allow right turns on red, but if you are going to have orderly traffic in order to avoid needless death, you will have to have some kind of decision about things like this.
In such cases, these decisions themselves have the authority of law, because they receive that authority or force of law derivatively. This gives us human-made law. It follows from this, of course, that human law can have no force of law unless it is consistent with natural law. We could easily put the matter in other terms. Nothing proposed by a human being can have obligatory force for others if it is not the sort of thing a rational person can accept as obligatory. But what determines what a rational person can accept as obligatory? The fundamental principles of practical reason, which are the basic precepts of natural law. We accept human laws as having force because reason shows that they should; and it does so by natural law. Another way to put it is that all human-made law must be a means to the common good of a given society; but the common good of any given human society includes the common good of all human beings, which is precisely what is articulated, explicated, and specified in natural law.
This has an interesting corollary that I think is not often taken as seriously as it should be. The most natural and fundamental form of human-made law is not statute but custom. In any given society, there are things you just don't do, whether or not anyone tells you that you can't, and in any given society, there are presumptions about what can be expected and what can't. Even when we have statutes, it is customary law that tells us how we should be interpreting and applying them. Different societies will have different customary laws, so that in one society custom might require that we construe a statute very broadly while in another society custom might require that we construe an apparently similar statute very narrowly; but no society can legally cohere except insofar as it builds on customary law.
There are large areas of life where customary law would suffice in principle, but in practice we need some common reference point that has to be more precise or more enduring than custom alone can be. One can go beyond custom in many different ways. (It should be noted, however, that in a sense no human law goes beyond customary law. Statutory laws get their force from natural law through customary law, so all statutory laws are just specific ways we continue the customs of our people, and even when they are brand-new or require a change of specific customs, they must at some level be continuing more fundamental customs of our society.) One possible way to start doing it is by common law. Anglo-Saxon common law is customary law given greater precision and organization by extensive court precedent; in a sense, it is a 'customary law of the courts'. Another way it can be done is by mutual agreement on the basis of shared customs. This is how the law of nations or international law grows up: on the basis of overlaps in their customary law, nations agree to abide by certain specific provisions, which become international law. Early discussions of jus gentium are very little more than discussions of customary law itself as it relates to nations; but as nations interact treaties, alliances, and agreements grow up that require more explicit statement. A third way it can be done is by legislation in the strict and proper sense. Citizens are the natural legislators of a given society, and they get together and create law, or else, for practical convenience and efficiency, they delegate their legislative authority to someone who creates law for them.
Another corollary of the principle that human-made law, from the barest customs to the most sophisticated regulatory systems and schemes, is subject to natural law, is that civil disobedience is sometimes legitimate or even obligatory. This is an important feature of natural law theory that has been part of it from the very beginning, because it follows from the fact natural law draws the line between legitimate authority and usurpation of power. Historically, natural law theorists have been very cautious about it, so that most natural law theorists have tended to argue that civil disobedience should usually be the civil disobedience of magistrates in the face of the usurpations of higher magistrates. However, there is no way to eliminate a more general kind of civil disobedience, because the natural magistrates of any society are the citizens themselves, and any other magistrates are merely delegates of the citizens at large. (This sounds very modern, but it is important to remember that this is the way natural law theorists talked even about monarchies and principalities: kings have authority from customary law, by means of which they are given the role of taking care of the common good of their people, who are nonetheless still the natural and primary caretakers of their own common good.) Thus laws that violate natural law have no particular authority and on their own need not be obeyed. It still may be necessary to obey them, but if so it will only be because disobeying them would also require violating natural law. For instance, a law may be illegitimate and without authority, but just given the accidents of circumstance, you can't disobey it without doing grave harm to innocents, whereas obedience doesn't do much harm at all. In these cases, though, obedience is just a matter of tolerating something in order to avoid worse things. In other cases, obedience may itself require violating natural law, and in these cases the law not only has not authority but must be opposed. The natural law approach to civil disobedience is laid out with admirable accessibility and practicality in Martin Luther King, Jr.'s Letter from a Birmingham Jail.
On the legislative side, one useful guideline for recognizing the bounds of legitimate legislation, the kind of legitimate that has the authority requiring civil obedience, is that given by Isidore of Seville in the Etymologies (5.21): "Law shall be virtuous [honesta], just, possible, consistent with nature [sometimes the phrase is punctuated so that the previous two are one item, 'naturally possible', and sometimes it is read as also combined with the next, so 'possible according to nature and national custom'], consistent with national custom, appropriate to time and place, necessary, useful; clear, lest by its obscurity it contain sophistry [captio, which can also mean 'deception' or 'trick']; not framed for private convenience but for civic common good [pro communi utilitate civium]."
A third corollary is the doctrine of toleration. It is simply not reasonable to try to handle every problem, every bad thing in society, by law, whether that law be customary or statutory. Human beings are imperfect, and you have to allow for normal failings and error. Further, sometimes it happens to be the case that enforcing a law, even one quite good in the abstract, would cause serious harm to others, in which case we should ask the question of whether the good of the law really merits doing this harm, and if the harm is serious, we shouldn't have that law, even if we still hope that people act as if it were in force. As Aquinas says (ST 2-1.96.2):
Human law is put forward for the human multitude, in which the greater part are not completely virtuous. And for this reason human law does not prohibit all the vices from which the virtuous abstain, but only the most serious, from which it is possible for the greater part of the multitude to abstain; and particularly those that are a nuisance for others [nocumentum aliorum], with regard to which, if not prohibited, human society could not be preserved, as when human law prohibits homicide and fraud [furta, which can also mean 'theft'] and such like.
Human law, then, must be such as can be reasonably applied without violating natural law, and this will require accepting in every society some imperfections that cannot be completely eliminated, but must simply be tolerated as the cost of continuing to exist as a society. Thus natural law governs human law, but not only does it not require utopianism about laws, it is actually inconsistent with it. This is ultimately due to the fact that natural law theory is a theory of practical rationality.
And that's the brief introduction. It could have been briefer and more introductory, because the basic idea of natural law is quite simple, but I find that people are impatient when it comes to natural law theory. Both critics and supporters often show by their actions that they don't actually care about the structure of the theory; they are only interested in rushing to whatever their pet topic is. This has always been a problem faced by natural law theory, although the topics change from society to society; in ours sexual topics tend to predominate, but there have been times and places when the all-devouring topics were economic or matters of international law. This is not a reasonable way to approach the matter, but it is inevitably human. Preventing misconceptions at this level requires getting into rather more complex issues than would be necessary if people were content to get a strong foundation in the basics and then slowly work out from there.
It is, however, brief and introductory. I haven't even looked at all the issues that are raised by Aquinas, whose questions on Law in the Summa Theologiae should be read closely by anyone who is interested in the subject. I haven't looked at the precise relation between natural law and a true commonwealth, which was Cicero's major concern in De Re Publica. While I've touched on a few historical disputes, I've steered clear of a number of important ones. For instance, Scotists and Thomists have considerable agreements in the area of natural law theory, since Scotus's account of natural law is very similar to Aquinas's, differing mostly in emphasis (largely due to the Franciscan/Dominican difference), but there are important disagreements between the two approaches, most particularly in the theory of dispensation of natural law (dispensation occurs when a law can be set aside for very rare but genuinely possible circumstances), which I've avoided entirely, because it is a very complicated issue. I've gestured at some of the classic texts, but haven't looked in detail at arguments or specifics. Anyone interested in natural law theory needs to have read Vitoria's De Indis, in which he sharply criticizes Spain's treatment of the native populations in South and Central America; King's Letter from a Birmingham Jail, which is mentioned above; and Maritain's Communication with regard to the Draft World Declaration of Human Rights (PDF). In addition, there are plenty of discussions, both from a classical and a New Natural Law perspective, of modern issues that are worth reading, whereas I have deliberately avoided the pet topics of our own day like the plague. No matter how important issues may be for a society or even in general, our view of reason itself, which natural law theory primarily considers, should not be forced to bow to the tastes and arbitrary assumptions of any particular Zeitgeist, but should be taken seriously in its own right first. Historically, the single most important issue in natural law theory was usury, and that is a complicated, and easily misinterpreted, history; but it is certainly the sort of thing that a serious student of natural law theory needs to look at. Many more issues could be added. Natural law theory is not the sort of thing that can be put in a short space: it is a massive tradition of inquiry into the most fundamental aspects of human life and practice. And that is a very big field indeed.