The best statement of Thomas's natural law reasoning is in his "Treatise on Law" in the Summa Theologica (I-II, q. 90-114). Within that section of the book, the most notable passage is question 94, article 2: "Does the natural law contain several precepts or only one?" Here Thomas sketches the general precepts of natural law as organized in three levels.
In my paper, I will argue that the content and structure of these precepts manifest human biological nature as presented in the biological science of Aristotle and Albert the Great (Thomas's teacher and mentor). I will also argue that this biological understanding of natural law is compatible with Darwinian evolutionary biology.
Here's my translation of one passage from this part of the Summa:
. . . The precepts of the natural law in a human being are related to action as the first principles to matters of demonstration. But there are several indemonstrable first principles. Therefore, there are also several precepts of natural law.
. . . The precepts of the natural law are related to practical reason as the first principles of demonstration are related to speculative reason. For both are self-evident principles. . . .
Now, there is a certain order to be found in the things that fall under human apprehension. For what first falls within our apprehension is being, the understanding of which is included in everything that one understands. And so the first indemonstrable principle is that the same thing cannot be affirmed and denied at the same time, which is based on the nature of being and nonbeing, and all other principles are based on it, as the METAPHYSICS says. And as being is the first thing that without qualification falls within apprehension, so good is the first thing that falls within the apprehension of practical reason. And practical reason is ordered to action, since every agent acts for the sake of an end under the meaning of good. Consequently, the first principle in the practical reason is one founded on the meaning of good, namely, that good is what all things seek. Therefore, the first precept of the natural law is that we should do and seek good, and shun evil. And all the other precepts of the natural law are based upon this, so that whatever the practical reason naturally apprehends as human goods or evils belong to the precepts of the natural law as things to be done or shunned.
And since good has the meaning of an end, and evil the meaning of the contrary, hence it is that all those things to which human beings have a natural inclination are naturally apprehended by reason as being good and, consequently, as objects of pursuit, and their contraries as evil and objects of avoidance. Therefore, the order of the natural inclinations is the order is the order of the precepts of the natural law.
Because there is first in a human being 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 existence according to its nature. And according to this inclination, whatever is a means of preserving human life and preventing the the contrary belongs to the natural law.
Secondly, there is in a human being an inclination to things that pertain to him more specially according to that nature that he has in common with other animals, and accordingly, those things are said to belong to natural law "that nature has taught all animals," such as the intercourse of male and female, the education of children, and so forth.
Thirdly, there is in a human being and inclination to good according to the nature of reason, which is proper to him; thus a human being has a natural inclination to know the truth about God and to live in society. And so 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 this inclination.
BEING AND GOODNESS
Thomas has said that while being and goodness are the same, they differ in that goodness appears under the aspect of desirableness, because the good is the desirable (I, q 5, a 1). Being and goodness are the same, because the good is that by which something becomes what it is by developing its potentiality to actuality, which is its perfection. Every living being has some tendency or inclination; it aims at an end or goal. The good of each living being is its self-fulfillment.
The growth of an acorn into an oak tree or a puppy into a dog illustrates this. It is good for a plant or animal to grow to maturity. It is bad for its growth to be impeded by unfavorable circumstances. Thus, we recognize a puppy to be defective if it cannot develop fully the potentialities of its species. Goodness is not some external standard imposed on things from the outside; it is rather the unfolding of the innate tendencies of things. It is therefore self-evident that the good is to be sought, because by definition the good is what each thing seeks. To say that all things should seek the good is simply to endorse what all things strive to do anyway.
While speculative reasoning apprehends the being of things, practical reasoning apprehends the being of things under the appearance of their desirability or undesirability. Thus, practical reasoning is not a matter of pure logic, because it requires a combination of the rational and the inclinational, so that things are apprehended practically in relationship to one's desires as either facilitating or impeding one's inclinations. Consequently, all those things to which we have a natural inclination are apprehended by reason as being good. The order of our natural inclinations then set the order of the precepts of the natural law.
Thomas distinguishes three levels in the natural inclinations. But from what he says elsewhere in the Summa, we can add a fourth level. As influenced by Aristotle's scientific theory of biological inheritance (GA 767b24-69b31), Thomas saw three levels of the human biological desires. These desires are "generic" as shared with other animals, "specific" as shared with other human beings as rational animals, or "temperamental" as showing the individually unique traits of a particular human being (ST, I-II, q 10, a 1, ad 3; q 46, a 5; q 51, a 1; q 63, a 1). There seems, then, to be four levels of natural inclinations: substantial nature, generic nature, specific nature, and temperamental nature.
Following the teaching of the Neoplatonic Book of Causes, Thomas sees the first level of reality as the creation of "being" (ens) or "existence" (esse). All substances seek the preservation of their being. Human beings share in this most fundamental level of reality in their natural inclination to survival. This corresponds to the natural biological desire for self-preservation.
Because of this natural desire for self-preservation, it belongs to natural law that killing in self-defense is justified, while unjustified killing of another human being is punished as murder (I-II, q 100, a 8, ad 3; II-II, q 64, a. 7).
As animals, human beings share in the natural inclinations of other animals, and particularly the mammalian animals that reproduce sexually and invest care in their offspring. In explaining this animal level of natural law, Thomas quotes from the ancient Roman jurist Ulpian: "Natural right is that which nature has taught all animals." Here the term "natural right" (ius naturale is interchangeable with "natural law" (lex naturalis). To illustrate the natural inclinations that human beings share with other animals, Ulpian referred to the sexual intercourse of male and female and the parental care of offspring as animal propensities that sustain human marriage and family life in conformity to natural law. Quoted at the beginning of Justinian's Institutes (533 A.D.), Ulpian's remarks entered the medieval tradition of natural law reasoning, and they were cited by Thomas when he explained natural law as rooted in the natural inclinations or natural instincts that human beings share with other animals. Each species of animal has a natural law corresponding to the natural inclinations of the species. The natural law for human beings is similar to the natural law of those animals who need to engage in the mammalian bonding between male and female and between parent and offspring. "Natural right in the strict sense" applies only to this level of common inclinations shared by human beings and other mammals (I-II, q 91, aa 4, 6; q 95, a 4, ad 1; II-II, q 57, a 3; Suppl, q 65, a 1, ad 4).
Thomas observes that unlike plants and inanimate entities, human beings and other intelligent animals display reason and desire in their movements (I, q 78, a 4; q 80, a 1; q 83, a 1; I-II, q 6, a 2; q 40, a 3; q 58, a 1; Commentary on Aristotle's "De Anima", secs. 629, 874). Intelligent animals consciously apprehend the objects of their desires, gather and assess information in their environment relevant to their desires, and then act according to their judgment of how best to satisfy their desires. In doing this, they learn to apprehend physical things and other animals as pleasurable or painful, useful or harmful, friendly or hostile. They act voluntarily in that they initiate acts as guided by some knowledge of their goals. They remember the past and anticipate the future. As social animals, they judge the intentions of other animals and communicate with them to act for common ends. They display and recognize social emotions such as love, hate, and anger. They learn from experience, and they transmit what they learn to others.
In so far as Thomas thus recognizes the continuity between human beings and other intelligent animals, he takes a position that is compatible with the Darwinian idea that the human species evolved from ancestral species of nonhuman animals, although Thomas never developed the idea of evolution. This supports Anthony Lisska's claim that Thomas's view of human nature and natural law is rooted in a "biological paradigm" (Aquinas's Theory of Natural Law: An Analytic Reconstruction [Oxford: Clarendon Press, 1996], pp. 68, 96-109, 131, 189-91, 198-201, 218-22, 258).
The biological character of Aquinas's reasoning about natural law as rooted in natural desires is clear in his account of marriage and familial bonding. He speaks of the human disposition to marriage as a "natural instinct of the human species" (SCG, bk. 3, chap. 123). On his account, the primary natural end of marriage is to secure the parental care of children, while the secondary natural end is to secure the conjugal bonding of male and female for a sexual division of labor in the household.
Among some animals, Aquinas observes, the female can care properly for her offspring on her own, and thus there is no natural need for any enduring bond between male and female. For those animals whose offspring do require care from both parents, however, nature implants an inclination for male and female to stay together to provide the necessary parental care (SCG, bk. 3, chaps. 122-23).
Just as is the case for those animals whose offspring could not survive or develop normally without parental care, human offspring depend upon parents (or parental surrogates) for their existence, their nourishment, and their education. To secure this natural end, nature instills in human beings natural desires for sexual coupling and parental care. Even if they do not have children, however, men and women naturally desire marital union because, not being self-sufficient, they seek the conjugal friendship of husband and wife sharing in household life.
Marriage as constituted by customary or legal rules, Aquinas says, is uniquely human, because such rules require a cognitive capacity for conceptual reasoning that no other animals have. Even so, human rules of marriage provide formal structure to natural desires that are ultimately rooted in the animal nature of human beings. Aquinas explains the natural laws of marriage in the light of Aristotle's biological studies of animal sexuality, reproduction, and parental care (compare Aquinas, SCG, bk. 3, chaps. 123-124; and Aristotle, HA, 571b3, 608b19, 610b35, 613a6, 613b33, 617a10).
The third level of natural law is uniquely human because it corresponds to the uniquely human capacity for reason, which gives human beings a self-conscious awareness in deliberately formulating a plan of life for the fullest satisfaction of their natural desires over a complete life. This includes not only social desires for living in society but also "the natural inclination to know the truth about God," which corresponds to what I have identified as the evolved natural desire for religious understanding.
Aquinas believes that the natural human desire to understand the causes of all effects leads to the desire to understand the ultimate causes--the uncaused causes--of everything, which can only be finally satisfied by the eternal contemplation of God in Heaven. The way to this supernatural happiness requires revealed religion and divine law, which surpasses natural law.
But even at the level of natural law, there is a "natural instinct" for reverence that supports natural religion. Moreover, it is natural for human laws to support religion in so far as it contributes to the moral order of human life (ST, I, q 12, a 1; I-II, q 3, a 8; q 99, a 3; II-II, q 81, a 5; q 85, a 1; SCG, I, 11; III, 25, 50-51, 57, 119).
Notice that the natural inclination "to know the truth about God" is not necessarily a natural inclination to know God. The "truth about God" might be atheism. This points to Aquinas's distinction between natural reason and supernatural revelation. Faith in revelation cannot be a matter of rational demonstration. So the natural inclination "to know the truth about God" seems to be the natural inclination to know the ultimate causes of things, which supports the life of philosophy or science. That's why the natural law precept following from this natural inclination is "shun ignorance."
Notice Thomas's remarkable silence about revelation in this article of the SUMMA. In this article, there is not a single reference to the Bible. The desire "to know the truth about God" is identified as a natural inclination requiring natural reason guided by the natural precept to "shun ignorance." From the point of view of natural law, it seems, "knowing the truth about God" is an exercise of philosophy or science in investigating the ultimate causes of things, which does not depend upon faith. Here we see the contrast between reason and revelation.
"To have faith is not in human nature," Thomas insists (II-II, q. 10, a. 1, ad 1). To "know the truth about God" cannot be faith, because faith is beyond natural reason and can only come by a supernatural infusion from God (I-II, q. 104, a. 1, ad 3; II-II, q. 6, a. 1; q. 8, a. 1). Of those who see the same miracle or hear the same sermon, some will believe, and others will not. Natural reason by itself could never lead us to have faith, because faith must ultimately be a supernatural gift from God.
Although Darwinian science can neither confirm nor deny the supernatural truth of revelation and divine law, Darwinian science can explain natural religion as an expression of the evolved natural desire for religious understanding. We can see the evolution of religion in the evolved tendency of human beings to project their experience of mental intentionality and their "theory of mind" onto the universe as they move beyond nature to nature's God. We can also see how such religious belief reinforces the moral order of a human community.
At the level of temperamental nature, each human individual is unique in the innate dispositions that constitute individual identity. By natural temperament, "one man has a natural aptitude for science, another for fortitude, another for temperance, and in these ways, both intellectual and moral virtues are in us by way of natural aptitude, inchoatively, but not perfectly" (ST I-II, q. 63, a. 1).
So, for example, although sexual mating and parental care are natural inclinations for most human beings, some human beings are temperamentally inclined to refrain from sexual mating and parental care, and thus they are naturally suited for celibacy (ST, Suppl, q 41, a 2, ad 4).
This raises an interesting question. Aquinas condemns homosexuality as "contrary to nature," because the only natural sexual activity for human beings is heterosexual intercourse (ST, I-II, q 94, a 3, ad 2). But if homosexuality manifests a natural temperament of some individuals, who show a natural inclination for same-sex conjugal bonding, and if this does not hinder heterosexual marriage and parenting, could tolerance for homosexuality be warranted by natural law?
THE BIOLOGY OF THE SOUL
All of these levels of the natural inclinations can be studied by biological science because "the soul is united to the animal body" (ST, I, q. 76, a. 7). Even the "intellectual soul" of a human being is united to the human animal body, although the emergence of the human "intellectual soul" in the body of the human embryo requires a special infusion from God (ST, I, q. 116, a. 2).
Aquinas suggests, however, that this miraculous intervention by God in embryological development cannot be known by biological science, because it depends on faith, which cannot be rationally demonstrated. "Faith and reason are not about the same things," and therefore "the reasons employed by holy men to prove things that are of faith are not demonstrations" (ST, II-II, q. 1, a. 5). Thus, there remains an irreconciliable tension between science and faith, reason and revelation.
This explains why Pope John Paul II, in a statement to the Pontifical Academy of Sciences in 1996, accepted evolutionary science, while also insisting that "the spiritual soul is immediately created by God."
Considering what Aquinas says about the natural law of knowing the truth about God and shunning ignorance in the exercise of natural reason, we might wonder whether the reliance on faith rather than science violates natural law.
Aquinas does say, of course, that the ultimate end of human life is the supernatural happiness of the afterlife. But he is clear that this does not belong to natural law, because this supernatural realm can be known only by divine law. The moral and intellectual virtues that perfect our natural inclinations according to natural reason depend on purely natural human experience. Natural law is not directed to a supernatural end. Consequently, the theological virtues by which human beings are directed to a supernatural end must be directly infused by God. "The power of those naturally instilled principles does not extend beyond the capacity of nature. Consequently, man needs in addition to be perfected by other principles in relation to his supernatural end" (I-II, q. 63, a. 3, ad 3).
Is Aquinas covertly taking the side of scientific reason against biblical revelation, even as he promotes the most rational interpretation of biblical revelation because of its moral benefits?
Some posts on related themes can be found here, here, here, here, here, here, here, and here.
The Pope's statement can be found here.