Because in man there is first of all an inclination to good in accordance with the nature which he has in common with all substances: inasmuch as every substance seeks the preservation of its own being, according to its nature: and by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law. Secondly, there is in man an inclination to things that pertain to him more specially, according to that nature which he has in common with other animals: and in virtue of this inclination, those things are said to belong to the natural law, "which nature has taught to all animals" [Pandect. Just. I, tit. i], such as sexual intercourse, education of offspring and so forth. Thirdly, there is in man an inclination to good, according to the nature of his reason, which nature is proper to him: thus man has a natural inclination to know the truth about God, and to live in society: and in this respect, whatever pertains to this inclination belongs to the natural law; for instance, to shun ignorance, to avoid offending those among whom one has to live, and other such things regarding the above inclination.
Because of this some people come away with the view that the way you determine a precept of natural law is to start by identifying a natural inclination, and somehow get the precept from the character of the inclination. This, however, is wrong. St. Thomas does not say that we discover the precepts by looking at the natural inclinations. On his view, we discover them by either (1) deriving them from the first precept of natural law (which follows from the first principle of practical reason), "Seek good and avoid evil," or (2) by resolution, i.e., by seeing how various candidates fit with the already established precepts. What Aquinas tells us is that the precepts of natural law are ordered according to the order of the natural inclinations, and this is something different again. The reason why the precepts of natural law are ordered according to the order of the natural inclinations is that natural inclinations are the way we recognize, so to speak, various things as good. Practical reason tells us how these goods relate to each other, and the pursuit of these various kinds of goods is made to be orderly by natural law. If we start with the actual things we do on the basis of natural inclinations, we are faced with two possibilities:
(1) The natural inclinations are, in the action, properly ordered to each other, in which case we are already looking at the natural inclinations recognized as being set in order by natural law.
(2) The actions following from these natural inclinations are not already recognized to be ordered by natural law, in which case we can learn nothing about natural law from them, because we cannot rule out that the actions resulting from them are defective, excessive, or twisted in some way.
Thus either we already know what's in accordance with the precepts of natural law or we can't derive the precepts from the natural inclinations. In the former case we could still learn something (e.g., better ways to formulate or express what we already know), but this is not something new about natural law itself. The natural inclinations themselves do not tell us how they should be ordered to each other (you can't just look at the inclination to self-preservation and formulate a precept, because you have to take into account all the other natural inclinations and how they are related to self-preservation -- but the inclination to self-preservation itself tells us nothing about this); but we cannot accurately formulate the precepts of natural law except by determining how the inclinations should be ordered to each other.
But the properly ordered natural inclinations are the natural inclinations as ordered by natural law; so it makes sense to think of the order of the precepts of natural law in terms of the order of the natural inclinations. They both follow from how different kinds of goods are ordered to each other. The inclinations are the seeds of pursuing the goods in this order and the precepts identify how these goods may pursued without violating that order.
