Having considered the way in which natural law theory is a theory of practical reason and the status of the principles of practical reason as law, we have to say something about the features of natural law itself, of which the most important are the ways in which the precepts of natural law are known, the ordering of those precepts according to the ordering of goods, and the reasons for deviations from natural law and disagreements about its precepts.
IIIa. Particular Precepts as Known
Since natural law consists of the fundamental principles of practical reason and their application to particular human goods as law, there are necessarily two aspects to how precepts of natural law are known. On the level of fundamental principles, the precepts of natural law have to be self-evident in the way any fundamental rational principles are, i.e., understood to be true when truly understood. The primary example of this, of course, is that of the first precept, good is to be done and sought and bad avoided, without which it is not possible to consider any plan or decision in a rational way at all. There is, however, no reason to think that all the fundamental principles of practical reason reduce completely to this one precept, any more than there is reason to think that all the fundamental principles of speculative reason reduce completely to the principle of noncontradiction. The primacy indicates its universality of application and its basicness in the order of practical consistency, just as it does if we consider the principle of noncontradiction in the order of theoretical consistency; it doesn't indicate that it's the exclusive or only precept of natural law that can be understood on its own terms. Indeed, given the way in which Aquinas argues for natural law, it is clear that any self-evident logical principles that can in any sense be said to apply to the relation of means to ends are themselves self-evident principles of practical reason. Likewise, there will certainly be precepts that identify self-evident features of practice itself.
Just as with speculative reason, however, we have to be careful even here. There are many logical and mathematical truths that are self-evident, in the sense that they are understood to be true when understood truly, but this does not mean that understanding them truly is always an easy endeavor. The most general and overarching principles of logic and mathematics can be understood by anyone capable of human thought; but as they increase in complexity or sophistication, the sphere of those who can understand them easily shrinks dramatically. Further, the histories both of logic and mathematics show that understanding in either field may take an extraordinary amount of careful reasoning. This is true of practical reason, as well. The first precept and other very basic precepts are easily intelligible and are recognizable whenever people consider whether plans or decisions are good or bad, reasonable or unreasonable. If one understands what decisions or plans are, or what means and ends are, or what goods and actions are, one already implicitly recognizes them. Many principles of practical reason, however, may be much more difficult to grasp or understand, and require greater preparation and study before one can do so.
The difficulty can be seen to increase when we consider the fact that they must be applied to particular circumstances. The range of circumstances to which human beings are exposed is immense, our previous understanding of the precepts may not be sufficiently good to determine what to do under some possible conditions, and (as we will see a little farther on) many goods and bads must be considered in almost any situation. This means that application will rarely if ever consist of straight deduction from general principles, the way it often will in theoretical reasoning. Both ceteris paribus and mutatis mutandis will often need to be assumed even if they are not explicitly stated. A single variation of circumstance can sometimes shift the kinds of goods being considered in a particular case, and thus the principles that are relevant. Thus, to use Aquinas's example, most cases of returning what has been entrusted to you are cases where it is clear that returning it is the right thing to do; however, when you add some additional circumstance, like causing great harm to the people around you, it is no longer so clear, not because the general principle of returning what is entrusted was wrong, but because it may no longer be clear how to apply it (e.g., whether it really requires one to return it at that time or under thsoe conditions), or even, if the circumstances are different enough, whether it still applies at all (e.g., whether the person has done something that abrogates any obligation in the matter).
This fits with our usual experience of practical reasoning, in which it is more a matter of estimation than rigorous deduction. This does not mean that there is no logic to it, or that these estimates cannot be evaluated as good or bad under the circumstances. Just as in practice we must often make do with mathematical estimates, which can nonetheless be evaluated as good or bad according to more rigorous mathematical standards, so too in practice we must often make do with practical estimates about how good or bad, and in what ways, particular actions will be in particular circumstances, while still being able to evaluate our estimates by more rigorous reasoning. To understand more fully how this works, we need to look at how precepts are ordered and the things that can interfere with good practical reasoning, which we will do in a different post.