If you are a philosophical naturalist (="nature" is all that there is), then what sense do you make of ethics? The Boston Review has a nice, well-written article called "Knowing Right and Wrong," written by MIT philosopher Alex Byrne.
Byrne asks, is boiling babies for fun wrong? It's not an easy question to answer if one is an atheist. This is because "Everything, in short, is a natural phenomenon, an aspect of the universe as revealed by the natural sciences. In particular, morality is a natural phenomenon. [Therefore] Moral facts or truths—that boiling babies is wrong, say—are not additions to the natural world, they are already there in the natural world, even if they are not explicitly mentioned in scientific theories."
On philosophical naturalism there is no "ought." Thus, the statement "We ought not to boil babies" is cogitively meaningless. This idea is rooted in Hume, who showed us that via the senses one cannot "see" metaphysical things such as causality. It seems empirically true that babies suffer in boiling water, but one cannot see an "ought not" in their pain. So on the face of it it seems that philosophical naturalism cannot say that we ought not to boil babies in water for fun. If "ought" is a fact, it is an "entirely different" sort of fact from facts of nature. "As the philosopher Simon Blackburn puts it in his Ruling Passions, “the problem is one of finding room for ethics, or of placing ethics within the disenchanted, non-ethical order which we inhabit, and of which we are a part.”"
Byrne then turns to meta-ethical issues; viz., issues concerning the foundation of ethics (how is ethics possible?). "The task before us is to try to squeeze morality into the “disenchanted” natural world; as Blackburn says, this “is above all to refuse appeal to a supernatural order.” "
Byrne mentions divine command theory. But it is noteworthy that he does not deal Yale philosopher Robert Adams's "modified divine command theory."
Byrne takes us on a philosophical tour through Moorean non-naturalistic ethics to emotivism to Mackie's meta-ethical nihilism to evolutionary ethics. Byrne concludes that "if there’s no room for ethics in a disenchanted nature, most of our distinctively human form of life is also excluded."
I agree. But I do not see that Byrne has made room for ethics in his philosophical naturalism.
Byrne's nicely-written essay, in the end, does not satisfy in its attempt to affirm that moral facts are really natural facts "in disguise." Byrne says that a "moral 'ought'" does not follow from a "natural 'is'." But he thinks a naturalistic 'ought' does follow from 'is' (i.e., nature). This is because, according to Byrne, 'ought' is a kind of fact of nature. Boiling babies IS "wrong," just not in any "moral" way.
[NOTE: Adams's modified divine command theory states that: "God is the source of morality, because morality is grounded in the character of God. Moreover, God is not subject to a moral law that exists external to him. On the Modified Divine Command Theory, the moral law is a feature of God’s nature. Given that the moral law exists internal to God, in this sense, God is not subject to an external moral law, but rather is that moral law. God therefore retains his supreme moral and metaphysical status. Morality, for the modified divine command theorist, is ultimately grounded in the perfect nature of God."
Thus Adams believes the two horns of the Euthyphro Dilemma are avoided.]