There's a trend among religious apologists to lament the supposed moral bankruptcy of non-theistic world views, saying that in the absence of God there is no way to account for moral ontology. And yet, these apologists seem to never actually engage with the ideas within contemporary metaethics. Indeed, they often seem entirely ignorant of what philosophers actually believe about moral reality, let alone why.
I seek to defend moral naturalism, a non-theistic account of moral ontology, against all competitors. Moral naturalists believe facts about morality, that is facts about irreducibly categorical norms, are identical to or constituted by some domain of natural facts. Most often this domain is facts about the flourishing and languishing of sentient beings. That being said, not all naturalists think a full and extensive account of moral ontology can be cashed out. We can be sure that the vast bulk of moral reality is constituted of facts about flourishing and languishing, even if there might be some problems at the fringes of moral experience. As long as the naturalist is aiming at merely identifying constitutive features of morality, and not a complete identity, counter examples lose their bite as long as they are few and far between, at the fringes of our moral experience.
The Conceptual Link
When deliberating what they should do people often forgo moral predicates altogether, and simply talk about the natural features of an action or situation. That it's perfectly natural to talk about morality without ever explicitly mentioning moral terms strongly suggests an intimate connection between moral concepts and the relevant natural concepts. Likewise, people often ask of themselves or others rhetorical questions like "what could it hurt?" That these questions are in fact rhetorical and not intended to be open ended for answering further goes to shows this strong conceptual connection. It is natural for people to speak as if, in the absence of anything that could be hurt or harmed by ones actions, there is not even conceptual possibility that those actions would be wrong. Now, I wont go as far to say that moral language can be reduced to natural language, I agree with the consensus of contemporary philosophers that G.E. Moore put analytic reductions of moral language, natural and non-natural, into the grave. Nevertheless, there is still a clear and resounding conceptual link between the two, which lends a great deal of credence to moral naturalism, and away from every other view.
The Supervenience of Morality on Nature
It's not controversial that moral facts seem to supervene on natural facts. That is to say it seems impossible for there to be a change in the moral features of some action or situation without there likewise being a change in the natural features. On any other view of moral ontology this supervenience is a highly mysterious coincidence. Why would the two be so intertwined across all possibilities as they are? Moral naturalists have an answer. Moral facts supervene on natural facts because moral facts are natural facts. Suppose that after every epic battle between Superman and some invading alien race, Clark Kent came into work the next day all cut up and bruised. After a while, you would start to suspect that Clark Kent just was Superman, that the two were the same person. Similarly, that any change in the moral facts necessitates a change in the natural facts seems to strongly suggest that moral facts are identical to or constituted by natural facts.
The Scientific Advantage
Because moral facts are identical to or constituted by natural facts, we are able to study them with the natural sciences. In fact, the moral naturalist would maintain that scientists have been studying morality for a very long time within the fields of biology, health sciences, psychology, sociology, economics and so on. We study what causes humans (and other sentient animals) to languish; what causes them to become sick, to suffer, or to die. We learn about morality when we study how different economic or government systems promote human flourishing, and which ones lead to languishing. What sorts of societies promote human flourishing. Just as the ancient Greeks studied the morning star, not realizing that their hypothesis and theories were likewise about the evening star, modern day scientists study the flourishing and languishing of humans (and other sentient animals), and thereby discover moral truth whether they realize it or not.
Evolutionary Psychology and Escaping Moral Skepticism
Evolutionary psychology offers us a compelling explanation of why we make the moral judgments we do: because these judgments drove our ancestors to act in ways that were beneficial to their survival and reproductive success. In other words, we make the judgments we do largely because these judgments were adaptive for our ancestors. This explanation carries a great deal of prima facie plausibility, is incredibly parsimonious, and is to some degree amenable to empirical confirmation and falsification. In other words, it has all the markings of a proper scientific hypothesis. On these considerations alone, the adaptive account of human moral psychology is very attractive.
But this adaptive account threatens moral knowledge. As Sharon Street argues in her paper The Darwinian Dilemma, it seems there is no connection between the truth of a moral judgement and its adaptivness. This is quite unlike judgements about physical reality that, if in error, can get you eaten by a predator, or plummeted off a cliff, or dead in a multitude of ways. Moral error doesn't seem to threaten ones chances of survival or reproductive success. And so, given the adaptive account of human moral psychology, we should expect our moral judgments to be largely (if not entirely) false.
Naturalists will be quick to point out that, on moral naturalism, moral error does in fact threaten ones chances of survival. Animals who are significantly in error about what causes and constitutes their own flourishing will not be able to maintain it, and thus die at a greater frequency. So on moral naturalism we can expect our moral judgements to be more or less aimed at truth, even in light of an evolutionary origin. At the very least, we should expect our moral judgements to be more no error prone than our physical judgements.
It seems naturalists are the only ones who can embrace the adaptive account without falling into extreme moral skepticism. And so non-naturalists of all sorts are forced into rejecting the adaptive account, most without any alternative explanation in sight. Theistic views might fair a bit better than the rest, in that they can fall back on a theistic account of human origins. They might say that God is directly responsible for human moral psychology, either by forming it ex nihilio or artificially guiding human evolution. But then they are adding a great deal more to the explanation than is necessary. And this loss of parsimony comes at the cost of an explanation that isn't even amenable to empirical confirmation or falsification. Even if theists can achieve consistency, there is nothing attractive about this alternate explanation.
But even with this, the theistic account explains too much. If God guided the evolution of human moral psychology, then it's very strange that our moral judgements are, in many respects, so error prone. There is and has always been great controversy surrounding ethical topics, and in the present it manifests in topics like abortion, euthanasia, sexuality, racial equality, gender roles, and so on. And, because of this, some significantly large portion of us must be or have been in error about these very important topics. But moral error leads to wrongdoing, something God supposedly wants us to avoid. It's very odd, then, that God would get involved with human evolution but not, as it seems, finish the job. Again naturalists will point out that, just as naturalism wards off threats of skepticism, it still leaves enough room for moral error. After all, evolution by natural selection is a messy, imperfect process. It does not care about our epistemic situation, and selects only by survival. And so on the adaptive account it's not surprising that we would have a handful of controversial topics here and there, where humans are generally not well equipped for discerning moral truth.
And so moral naturalism out competes theistic accounts of moral ontology twofold, and all other non-natural non-theistic views once over. With this, moral naturalism stands as the best account of moral ontology, since it's compatible with (without falling into deep moral skepticism) our best account of moral psychology.
The Ontological Parsimony
Moral naturalism is without the spooky metaphysical baggage of non-natural and supernatural views. It looks to the natural world, rather than to the supernatural, to explain moral ontology and, in doing so, only commits us to the existence of things everyone believes in anyway: the sorts of things a scientifically informed world view would contain. One doesn't need to believe in gods, or platonic forms, or mystical forces to believe in moral reality. One need only recognize objective facts about the flourishing and languishing of sentient beings, to find a solid ontological foundation for good and evil.