Whenever someone mentions “natural law” there is an immediate confusion that usually arises due to the contemporary understanding of the word “nature.” In modern usage we associate the terms “nature” and “natural” with the “natural world,” which is to say, the universal laws of physics and biology and all of the mechanisms that take place on this level. We don’t attribute to the word “nature” anything specifically human. It is taken as a context for all life rather than as a distinctive characteristic of a given being.
But when the Church speaks of “nature,” and especially when it speaks of natural law, it is speaking very differently. This is because natural law teaches that every being has its own “nature,” and that this imbedded nature also corresponds to an imbedded “natural law” which tends the being toward the perfection of its specific nature. It follows then that the “nature” in question will always be different depending on whether we are talking about a vegetable, an animal, or the human person. What is according to the “natural law” for one category of beings may not apply to another, because they have different natures. This is why the tendency to imagine “nature” as mere biological necessity applying to all material beings in the same way is a drastic oversimplification. Rather, when we are concerned with human behavior, we are concerned with man’s specific nature and, more importantly, his last end toward which this nature tends to move.
For example, we might say that sexual desire is “natural.” If we make the mistake of taking “natural law” to mean “biological necessity,” then we might end up drawing the conclusion that promiscuous sex is according to the natural law, since we see it all the time in animals and in fact this behavior is necessary to many of them. But we cannot transpose this principle onto a different nature—for example, onto human nature. Sexual desire is still in accordance with natural law for human nature, but only insofar as it reinforces the being’s development toward its ultimate perfection. While for certain animal natures this entails promiscuous sex, for man it does not. Sexual desire is therefore “natural” to man in a very different way than it is “natural” to animals, because man has different faculties and a different perfection which he must realize. He has a different nature and so the natural law does not direct him in the same way as it would direct a vegetable; likewise, the vegetable is directed very differently than a fish or a bird.
In order to gain a proper perspective on this subject, we must return to a more comprehensive notion of law capable of taking into account a hierarchy of orders and contexts, and which can deal with the diversity of life we find in the world. In traditional terminology, we must return to the three orders of law: eternal, natural, human.