Thomas Aquinas's natural law teaching suffers from a fundamental incoherence. On the one hand, Aquinas distinguishes natural law from divine law, just as he distinguishes what we can know by natural reason from what we can know only by divine revelation. Although divine law can reinforce natural law for those who are religious believers, natural law can stand on its own natural ground, because it is comprehensible to human beings by their natural experience of their natural inclinations and natural reason, regardless of whether they are religious believers. On the other hand, however, Aquinas embeds his teaching on natural law within a Christian theological teaching in a way that suggests that natural law depends on divine law. But in that case, it seems that natural law is not truly natural. Even the term natural law suggests this by suggesting that there must be a divine lawgiver. Consequently, Leo Strauss and his students have criticized Thomistic natural law as a betrayal of classic natural right.
The Darwinian account of the natural moral sense fulfills Thomistic natural law by grounding it in evolved human nature without any necessity for a theistic cosmology. In doing that, the Darwinian account follows in a tradition of Socratic/Ciceronian skepticism about the theistic cosmology of intelligent design, a tradition that stretches from Cicero's De Natura Deorum to Hume's Dialogues Concerning Natural Religion.
One can see what's at issue here by noticing the one place in the Summa Theologica where Aquinas refers to Cicero's De Natura Deorum. Aquinas is arguing that we must see the world as governed by God when he writes:
Certain ancient philosophers denied the government of the world, saying that all things happened by chance. But such an opinion can be refuted as impossible in two ways. First, by observation of things themselves. For we observe that in nature things happen always or nearly always for the best, which would not be the case unless some sort of providence directed nature towards good as an end, which is to govern. Wherefore, the unfailing order we observe in things is a sign of their being governed. For instance, if we enter a well-ordered house, we gather therefrom the intention of him that put it in order, as Tullius says (De Nat. Deorum ii), quoting Aristotle. Secondly, this is clear form a consideration of divine goodness. . . . (I, qu. 103, a. 1)
Although Aquinas refers to Cicero hundreds of times in his writings, he refers to the De Natura Deorum only three times, and this is the only reference to the book in the Summa Theologica. That's surprising. Since this book is one of the most important statements on ancient theological cosmology, one might think that Aquinas would need to consider this book as he lays out his own theological cosmology. We might explain this, however, by noting that this book contains the most explicit and rigorous attacks on theological cosmology as based on intelligent design reasoning. Aquinas never defends his own theological cosmology against these attacks.
The radical character of the skeptical attack on religion in Cicero's book is indicated by the fact that Hume followed the style and substance of Cicero's book in his own Dialogues Concerning Natural Religion, which was published after Hume's death because of his fear of persecution. Hume's skepticism about natural theology and the intelligent design argument was continued by Darwin in defending his theory of natural evolution against the theory of special creation.
In the passage above, Aquinas is referring to a defense of Stoic intelligent-design cosmology in Cicero's book coming from Balbus (2.15-17, 87, 95). Remarkably, Aquinas is silent about the vigorous refutation of Balbus's reasoning coming from Cotta, a Roman priest who is also an Academic skeptic. Moreover, Aquinas never responds to Cotta's criticisms of the sort of intelligent-design reasoning that Aquinas himself adopts.
Aquinas restates Balbus's claim that the only alternative to explaining the world as intelligently designed by God is to say that everything happens by chance. But Cotta insists that there's another possibility--that we can explain nature's order as a product of nature itself in which things emerge by the spontaneous order of natural processes without any need to assume a divinely intelligent mind at work. "But not all things, Balbus, that have fixed and regular courses are to be accredited to a god rather than to nature" (3.24).
Furthermore, Cotta indicates, Balbus's intelligent-design reasoning assumes a ridiculous anthropomorphic analogy by which we assume that the whole world is an artifact that implies the existence of a divine artist. Aquinas refers to Balbus's house analogy, which Cotta ridicules:
He says, "If we saw a beautiful house, we should infer that it was built for its masters, and not for mice; so therefore we must regard the world to be the house of the gods." Assuredly, I should so regard it, if I thought it had been built like a house, and not constructed by nature, as I shall show that it was.
Cotta challenges Balbus to prove that we must explain the world as analogous to a human artifact. Like Balbus, Aquinas assumes the plausibility of this analogy without proof. When Aquinas sets out his proofs for the existence of God, one of the objections he considers is: "For all natural things can be reduced to one principle, which is nature; and all voluntary things can be reduced to one principle, which is human reason or will. Therefore, there is no need to suppose God's existence." Aquinas replies: "Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause" (I, q. 2, a. 3, ad 2). Thus, Aquinas assumes that our natural experience with human agency can be anthropomorphically projected onto the world as the work of a "higher agent."
Aquinas makes it clear that natural theology depends on the plausibility of this anthropomorphic analogy of intentional artifice: "In all things moved by reason, the order of reason that moves them is evident, although the things themselves are without reason. For an arrow through the movement of the archer goes straight towards the target, as though it were endowed with reason to direct its course. The same may be seen in the movements of clocks and all engines put together by the art of man. Now as artificial things are in comparison to human art, so are all natural things in comparison to the divine art" (ST, I-II, q. 13, a. 2, ad 3).
Cotta indicates the many problems with this anthropomorphism of intelligent-design cosmology. We know by natural experience how human minds work in executing human intelligent design. But we know nothing about how divine minds could work to create the world. Human minds are always embodied. But it's ridiculous to think that the gods have human bodies, with the mortality and other limitations that come with embodiment. We might say that the gods are disembodied minds. But then it's inconceivable how a mind could have any knowledge without bodily senses.
It's not clear that Aquinas ever responds to such criticisms. Of course, his response might be that our understanding of the Divine Mind requires faith in divine revelation, and "faith and science are not about the same thing," because "the reasons employed by holy men to prove things that are of faith are not demonstrations" (II-II, q. 1, a. 5). But if that's his response, that only confirms Cotta's complaint that intelligent-design cosmology is not based on rational proof but on religious belief.
The ancient Christian writer Lactantius observed: "Cicero was aware that the objects of men's worship were false. For after saying a number of things tending to subvert religion, he adds nevertheless that these matters ought not to be discussed in public, lest such discussion destroy the established religion of the nation" (Divine Institutions, 2.32).
Does this explain Aquinas's reticence--that it was not good to publicly discuss the flaws in intelligent design cosmology, because this might subvert religious and political order?
Cicero has Cotta indicate in De Natura Deorum that Plato and other philosophers had developed intelligent-design cosmology as a noble lie to support popular morality and political order, a noble lie that the philosophers knew to be a lie (1.30, 77). In his Academica, Cicero has Varro argue that when Socrates turned away from cosmology to the study of the human things, this suggested that cosmology has nothing to do with the human good (1.15). Similarly, Cotta suggests that natural right has nothing to do with the cosmos or the gods, because it depends on human social life and on a naturally instinctive conscience by which human beings judge what is right and wrong for human life (3.38, 85).
Darwin deepened this understanding by showing how the "conscience or moral sense" could be explained as arising from the naturally evolved spontaneous order of human instincts, human customs, and human judgments.
And yet Darwin recognized that religious belief could be important for reinforcing this natural moral sense. He also recognized that the natural human desire for ultimate explanation leaves us with a fundamental choice between nature and God as the unexplained ground of all explanation, and thus he left open the unresolved conflict between reason and revelation.
Recent research in the evolutionary psychology of morality confirms and deepens each of these points concerning the evolved instinctive basis of morality, religious understanding, and intellectual understanding.
A good article on Cicero, Aquinas, and natural law is Adam Seagrave's "Cicero, Aquinas, and Contemporary Issues in Natural Law Theory" (THE REVIEW OF METAPHYSICS, vol. 62, March, 2009, pp. 491-523). Seagrave's argues--correctly, I think--that one can see how Thomistic natural law need not depend upon Christian beliefs if one sees it as a continuation of Ciceronian natural law, which was based on an Aristotelian understanding of human nature. Seagrave does not say enough, however, about the crucial issue of Stoic theological cosmology. He points to "the Stoic cosmology which serves as the foundation for the natural law, a cosmology which had been discredited by Cicero himself in his DE NATURA DEORUM and has been widely discarded since Cicero's time." But Seagrave does not reflect on the point that Aquinas embraced a Christian version of the Stoic cosmology. Nor does Seagrave reflect on the possibility that the Darwinian rejection of an anthropomorphic moral cosmology fulfills Cicero's suggestion that natural law need not depend on such a cosmology.
Some previous posts on related topics can be found here, here, here, and here.