Here now we should investigate the natural law under the aegis of human participation in it; that is, mankind as recipient of the natural law. The moral law is indeed natural in two senses. First, it is natural insofar as it expresses what it is for humans to function normally in their nature. This is the ontological element. Second, it is natural in the extent that it is naturally known; known not through conceptual knowledge or discursive reasoning but rather via inclination (an inclinatus as the medievals would say). This is the epistemological element. This second aspect adds the weight of being infallible and indubitable to the law; further, even though the law is not known through reason it remains the case that it is reasonable. Maritain uses the term ‘connaturality’ to grasp this epistemological aspect, and to highlight that the law is congenial with human nature and thus is known through an intuitive grasp. In this manner, the law is neither innate nor a matter of listening to our conscience, as many modern philosophers would have it.
It may be asked where this moral law comes from, and this should be clear from what has already been said. More can be determined from the way in which it is known; there are three conclusions that can be drawn. First, natural law is only concerned with those principles known through inclination. Second, given that they are known through inclination, these principles cannot be given demonstrative reasons. This should not be taken as a sign of irrationality but rather as a greater sign of validity; these principles cannot be conceivably doubted. Third, this is the case because human reason has no part in deciding what these principles are, either in causing their existence or in causing them to be known. However, they are laws, and as Aquinas stated, all laws are results of reason. What reason gives the principles of the moral law then? The answer is that uncreated Reason, the reason of Nature Itself, is what both established the moral law (through creating human nature) and also which causes the law to be known through the inclinations of human nature, to which human reason can and ought to be attentive. Thus, moral theory, ethical theory, seeks to make discursively known what is already intuitively known via our natures.
We can see that the natural law constitutes the fundamental principles of practical rationality, of ethics that is, and it is such by nature. We say “by nature” because it could not be the case that a being would share human nature with the rest of mankind and not be bound by these principles. As already stated above, this is because it is these principles and precepts that direct us both to goods and the good. This follows from the perfective aspect of our nature being directed towards the good, as understood within the rational order. This further ensures that these principles are universally known. This universal knowledge is readily exhibited in the way we find ourselves disposed and inclined towards various goods, and these are precisely the basic goods that the natural law instructs us to pursue. And, as will be discussed below, the awareness of these principles is given propositional content through prolonged reflection on history, anthropology, and practice.
The second, and crucial, aspect of the natural law is the epistemological element. This is the law as known and understood by the practical intellect. It was said that the moral law is known via inclination, and thus the principles of this law are said to be self-evident as such. This relates to the practical intellect, since there is a priority in the objects of the intellect. Aquinas states that the first object of the understanding is being. Being, however, is convertible with goodness (as one of the transcendentals); thus, goodness is the first object of the understanding. And since all action seeks an end that has the nature of good, the first principle of practical reason is founded upon this good: all things capable of seeking seek the good. Thus, as Aquinas says, “the first precept of the natural law is that we should do and seek good, and shun evil.” This is not itself the natural law but rather the first principle, a preamble of sorts; the natural law is the out-flowing of this principle. Again, as Aquinas says, “all the other precepts of the natural law are based on that precept, namely, that all the things that practical reason by nature understands to be human goods or evils belong to precepts of the natural law as things to be done or shunned.” And given that we learn what is good by inclination, the regulations of the law (i.e. all that follows from the first principle) are also known via inclination. Thus, to know what is good we first look at what we are naturally inclined towards, then we determine whether it is conducive and fulfilling of our nature.
Mention should be given also as to the development of the understanding of the natural law. Since it is unwritten, mankind’s knowledge of the natural has accumulated and grown slowly over time. This is in relation with the development of the moral conscience or sentiments over time. Anthropology has an important role to play in this regard. Hence, it remains the case that there are true principles of morality that we have yet to come to fully grasp, which unfold as culture and tradition progress. Notice, though, that the knowledge is always from the inside: from analysis of human inclination and society. This does not mean, as some would have it, that human inclination and society create the moral law. Rather, these are the guides to what already exists. This distinction is a fine line to walk, but landing on one side or the other has significant consequences. We must remember that natural law is only first principles of moral life, and we always move from the general to the specific through an ongoing process of progressive recognition of the good. To this foundation of natural law, then, is added the full spectrum of virtue theory, which again finds root in Aristotle. It should be clear, however, that without a sufficient foundation, such as the one provided for on a natural law analysis, a virtue theory cannot stand. From it, however, we are able to build an intricate and vibrant fullness of a moral theory.