Natural law theory is, as noted in the previous part, a theory of practical reason; but what gives its name is that it is a theory of practical reason that is also a theory of law, a theory of practical reason qua law. Thus an adequate discussion of natural law theory requires a discussion of the account of law it gives, and to this we now turn.
While he is certainly not the first to say something that might be construed as suggesting the idea of natural law, a fairly reasonable starting point for the history of natural law theory in Western civilization is Cicero. In De Re Publica he gives what clearly has all the elements of a natural law theory, which is to say, an account of human practical reason based on principles natural to it that is also an account of law. It forms one of the key elements of Cicero's theory of a republic or commonwealth. Cicero notes that human beings are in some sense at a disadvantage in the world at large -- in his notable phrase, Nature has treated us more like a stepmother than like a mother; if it were not for one thing, we would be outmatched by our environment, and that one thing is understanding or reason. By this we are able to write and study the stars, and to develop philosophy and thus civilization, that is, civil society. While we are not thrown into civil society, it is in some sense natural for us to build such civil societies. And as civil societies are constituted by justice and laws, any discussion of civil society must consider what justice and laws are. In the course of the dialogue, one of Cicero's characters, Laelius, argues that expediency is not the foundation of justice or law, and thus not the foundation of a genuine republic or commonwealth. Rather, there is a more fundamental foundation for both that, transcending any utility one might gain from either, still has purchase on us, because it is based on that very reason or understanding that makes civilized life possible in the first place. This is a true law, which is the same as right reason and is in accordance with our nature. Unlike human law it is universal and immutable, and (as it is that law which enjoins upon us morality itself) there is no field of genuinely human action that is outside its jurisdiction. No senate and no popular will can override it, for any attempted dispensation is unjust and unreasonable; it stands above human action in the sense that all human action is judged by it. It is promulgated by the God, and anyone who disobeys it does violence to his own nature. Laelius's account is applauded and supported by Scipio, another major character in the dialogue; and the agreement of the two is a clear sign of Cicero's own good favor toward this account. Much of this comes from the Stoics, but we see it come together here in a shape that will be quite familiar through the centuries. St. Ambrose and St. Augustine both show the clear influence of it, but, of course, the most famous exposition of this view is that of St. Thomas Aquinas.
In the first question of the Treatise on Law in the Summa Theologiae, which is question 90 of the First Part of the Second Part, Aquinas gives a general definition of law: Law is "nothing other than a certain promulgated ordering of reason to common good by one who has the care for what is common" or, as it is often translated, "nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated". There are four basic elements of this definition.
Law has to do with reason. Law regulates and measures human action in such a way that action is encouraged or restrained. This is a rational function: all genuinely human action is measured and regulated on the basis of reason, so that reason can in some way be said to be its source; thus law has to do with reason, either immediately or derivatively.
Law is an ordering to common good. Reason organizes and regulates action according to means and ends, as we noted in the discussion of practical reason. The most general end with which reason deals is the totality of good in a good life, and this does not exclude (and, indeed, requires) that reason consider what is good for everyone. Law clearly has to do with human beings in society, and therefore must consider precisely this: what is good for everyone in common. Thus law is an ordering to common good. It is worth pointing out, I think, that this common good is not, as Aquinas understands it, an overarching collective good separate from all individual good. It is instead the kind of good people share, literally good in common, by virtue of their living in society. It is also worth being wary of assumptions that tend to get imported by the translation "the common good". There is no one common good; there are many common goods, as many common goods as there are communities. Some common goods are more inclusive than others, but it is impossible to talk about any common good without having established what society or community one is considering.
Law is from one to whom it belongs to care for what is common. Ordering to common good typically depends on a whole community working as a whole: it is the whole community working together that has natural and primary responsibility to order itself to the relevant common good. However, practically speaking (and with law we are always speaking practically) it is often not feasible for the community to work as a unitary authority. To get around this problem, assuming there is no natural caretaker for the whole community, the whole community chooses someone to act for the whole community. Since law is an ordering to common good, its source has to be someone who is in some way caretaker of what is common to everyone.
Law is promulgated. Laws exist in order to regulate and measure human action, but they must be applied to action in order for this to happen. Since human action has its source in reason, the way in which laws are applied to human action must accordingly be rational. That is, law must be communicated in a rational way to the people to whom it applies, so that their reason can take it into account. This can happen in many different ways, but it must happen.
If any of these four is lacking, we may have something that looks a lot like a law but is not actually a law, and has no force as a law; a point which will be of some importance. There are, however, many different kinds of laws.
One of these kinds of law is natural law, which is the law constituted by the basic principles of practical reason. Obviously these principles are rational. They concern how things are ordered to good as such, which means that they are concerned with both private good and common good; insofar as they concern common good, they fit that part of the definition of law. They are also clearly promulgated, that is, our reason is notified of them, because they are intrinsic to reason, and without them we cannot reason practically at all. Therefore that leaves simply the source to whom it belongs to care for what is common.
This gets into an interesting issue. Aquinas himself is quite clear that the source of natural law is God, on whom human beings depends; and because the whole human community depends on God, God is the natural caretaker of what is common to the whole human community. As he puts it, natural law is the human participation of the eternal law of God by which God providentially orders the whole universe; it is, so to speak, a parceling out of eternal law in human form specifically for human beings. However, the language of participation is much more intimate in its suggestion than merely God imposing a law on us. Aquinas is equally clear that human beings don't just have their specifically human parcel of the eternal law of divine providence; divine providence itself is parceled out to us, again in a specifically human form specifically concerned with human life. Each human being, as rational, is in a sense part of divine providence itself. We all have a share in divine providence so that we, as rational creatures, are in Aquinas's phrase, "provident for ourselves and others". Only creatures with reason and understanding can have this subordinate providentiality, this active participation in the very providence of God. Thus there is a definite sense -- a derivative sense, but still a definite sense -- in which each and every human being, as a rational person, is caretaker for what is common to all human beings.
The participatory authority of each human being accounts for a feature of natural law theory that often confuses people, namely, the fact that sometimes natural law theorists talk as if natural law theory crucially depended on recognizing the existence of God's eternal law and thus the existence of divine providence, but that sometimes natural law theorists talk as if natural law does not depend on any specific belief about God, even the belief that God exists, so that one can recognize the precepts of natural law as such even if one does not believe God exists. In actuality, both can be true. Any natural law theory that is genuinely Ciceronian, or Augustinian, or Thomistic in inspiration cannot be complete without taking into account the fact that natural law is a sharing of the eternal law. However, precisely because this is not a mere imposition but a participation, we can recognize that the precepts of natural law are laws simply from the naturally provident character of reason itself. Full understanding of this providential character, and the corresponding force of law, requires placing natural law in the context of God's providence, because human beings are secondary and not primary caretakers of human good; but human beings are each themselves, by virtue of their own reason, caretakers of human common good. Aquinas does not develop this point at great length, so it is difficult to say exactly how much emphasis should be placed on this; some Thomistically inspired natural law theorists prefer to emphasize the importance of putting natural law in the context of eternal law, while others prefer to emphasize the intrinsically legal character of the principles of practical reason. Both have foundation in Aquinas. It is also important to understand that from a strictly Thomistic perspective this is a purely epistemological question of whether one can recognize natural law as law if one does not believe in divine providence; Aquinas affirms principles that imply that one can, and this has some confirmation in actual human behavior. But even if it weren't so, principles of practical reason have the force of law, because God exists and is caretaker of human common good, indeed all common good; and, likewise, even though it is so, any complete explanation of why the principles of practical reason can have the force of law will ultimately trace back to the fact that divine providence has the force of law, or to put the same matter in different terms, that human beings are subordinate caretakers of human common good sharing in the primary caretaking of God. This parallels the way Aquinas understands God's causal activity generally, and the parallel is not accidental.
This gives us the general understanding of what natural law is, but we need to look at features that are specific natural law, and then look at how natural law relates to another important kind of law, human or positive law.