Up until recently, I had thought that natural law theory was compatible with moral arguments formulated as follows:

1. If God does not exist, then objective moral values and duties do not exist.
2. Objective moral values and duties do exist.
3. Therefore, God exists.

Moral arguments of this kind have been made popular by defenders such as CS Lewis and William Lane Craig, and this specific formulation comes from the latter. In a post from a few years ago I explained my position on the compatibility of this with natural law theory as follows:

I think technically we can still use [the argument] as [formulated above], but we must recognise that it is partly dependent upon something like the fifth way for its soundness. At the end of the day I think much moral debate can be had without reference to God, since it is based on what is knowable about our nature. But ultimately I think any viable ethics depends on God, including natural law. (section 4.1)

This is admittedly not giving much credit to the argument, but I have since realized that even this weak support for the moral argument is misplaced. It seems to me that once we clarify the above formulation, the first premise will be seen to be incompatible with natural law theory, or at least some increasingly popular versions of it.

To start on the more technical side of things, the first premise should be understood as a non-trivially true counterfactual with an impossible antecedent (see here for details):

1′. If God did not exist, then objective moral values and duties would not exist.

So far there is still no obvious incompatibility with natural law theory, but we can go further. Presumably, if we are running this argument, then we think that there is something special about moral values and duties that calls out for a theistic explanation. That is, we are not interested in the general fact that anything whatsoever exists, but particularly the fact that moral values and duties exist. If this were not the case, then wouldn’t really be running a moral argument at all, but would instead be running a cosmological argument.

The point of the first premise, then, is that we finite agents are not sufficient to account for objective moral standards, and so the presence of such standards would imply the existence of God. This suggests that another way of stating the first premise is as follows:

1*. If we were to exist without God, then objective moral values and duties would not exist.

(Those of us who are convinced that God is required to account for any existence should also read this as a non-trivially true counterfactual with an impossible antecedent.)

Apart from the reasoning that got us here, further confirmation that (1*) captures the intent of (1) comes from how the premise is often defended. Consider, for instance, the following quote from Craig:

If there is no God, then any ground for regarding the herd morality evolved by homo sapiens as objectively true seems to have been removed. After all, what is so special about human beings? They are just accidental by-products of nature which have evolved relatively recently on an infinitesimal speck of dust lost somewhere in a hostile and mindless universe and which are doomed to perish individually and collectively in a relatively short time. Some action, say, incest, may not be biologically or socially advantageous and so in the course of human evolution has become taboo; but there is on the atheistic view nothing really wrong about committing incest. If, as Kurtz states, “The moral principles that govern our behavior are rooted in habit and custom, feeling and fashion,” then the non-conformist who chooses to flout the herd morality is doing nothing more serious than acting unfashionably. (William Lane Craig, The Indispensability of Theological Meta-Ethical Foundations for Morality)

Notice that this line of argument envisions a world where we exist without God, and puzzles over where moral values and duties are supposed to come from in such a world.

Now, while natural law theory may not pose any obvious problem for (1) or (1′), once we recognize that these amount to (1*) the problem becomes clear. The whole burden of a natural law theory is to ground moral truths in the natures of things, and having the nature that we do is part of what it means for us to exist. In the world described by (1*), then, the fact that we still exist with natures means that we still have objective moral duties and values even though God is not in the picture — at least from the perspective of natural law.

Of course, the exact details of this will differ depending on the version of natural law theory we consider. On Platonism these natures will be unchanging Forms in some third realm, on Aristotelianism they are intrinsic teleologies in things, and the new natural lawyers focus more on the nature of practical reason than on the natures of things. And each of these has variants within it. Some versions of Platonism equate the Forms with divine ideas, so that taking God out of the picture will take out natures with him. But other versions have God completely separate, meaning that natures stay even after God is removed.

Thomistic natural law theory is of the Aristotelian variety and is the version I find most compelling. On the one hand, it agrees with Aristotle that morality is fundamentally grounded in the intrinsic teleology built into us by virtue of the natures we have. On the other hand, contrary to Aristotle, it says that this intrinsic teleology still depends on God. Mind you, not in a way that makes it distinct from our nature, as if our teleology could in any way be separated from what we are. Rather, it is by creating and sustaining us as the kinds of creatures we are that God upholds the intrinsic teleology that fundamentally grounds morality. Of course, the details of this are quite complicated, but the point is that on the Thomistic view our intrinsic teleology is not mutually exclusive with God being the cause of our nature.

This brings us back to (1*). This premise asks us to consider the world where per impossible God does not exist and yet we still do. Because in such a world we still exist, we also still have natures and the intrinsic teleology which fundamentally grounds morality. This remains true even if our natures arose through blind evolutionary processes since what’s important is the nature we have, not how we got it. So, in this world where we exist without God there is still the foundational morality that arises from the natural law: it is still wrong for us to lie, to murder, to steal, etc.; we still have categorical obligations, are held accountable, and have a basis for moral authorities (see section 2.4 here); we still have objective virtues and vices; actions are still objectively good and bad. Of course, there will be no duties arising from divine commands, but on natural law theories, these are in addition to the natural law, not instead of it.

So, then, for those of us who accept the Thomistic account of natural law, the moral argument we’re considering should be rejected as unsound. And I suspect the same would be true for some other versions of natural law theory, whether they be Platonic, Aristotelian, or from the new natural lawyers. It is certainly true for Aristotle’s own version, which doesn’t even construe God as the cause of our intrinsic teleology. On the other hand, there is also a lesson for those defenders of the argument who don’t accept any of these natural law accounts: a full defense of the first premise requires a thorough critique of these different natural law theories, which is no simple task. Certainly not as simple as the quote above appears. After all, natural law theories have a long pedigree in the history of Western thought.

While this objection doesn’t affect all moral arguments, it is noteworthy because the version it does affect is quite common. The argument might still have apologetic value insofar as it could convince someone who already rejects natural law, but such a rhetorical strategy makes me somewhat uneasy.