Moral naturalism holds that moral properties and facts are nothing over and above natural properties and facts investigated by science. 'Right action' might reduce to 'action that promotes well-being,' and 'wrong' might reduce to 'causes suffering.' This avoids postulating mysterious non-natural moral properties while preserving moral objectivity.
Compare to scientific reduction: heat was reduced to molecular motion; water was reduced to H₂O. Consider whether something similar could work for morality, or whether 'good' resists reduction in a way 'heat' doesn't.
You already know from moral cognitivism that moral claims are truth-apt—that when someone says "slavery is wrong," they are making a genuine assertion that can be true or false, not merely expressing an emotion. Moral naturalism accepts that conclusion and pushes one step further: what kind of facts make moral claims true? The naturalist's answer is that moral facts are just natural facts, the same kinds of facts studied by empirical science. Nothing spooky or supernatural is required to make "causing suffering is wrong" true; it is true in virtue of facts about suffering, well-being, and the structure of human flourishing.
The most useful analogy is scientific reduction. Before chemistry, "heat" seemed to name something irreducibly special—a mysterious property objects possess when they feel warm. Chemistry identified heat with mean molecular kinetic energy. This was not a mere linguistic convention; it was a genuine discovery that heat just *is* a certain kind of molecular motion. Moral naturalists propose the same structure for ethics: moral reduction holds that "right" just is "promotes well-being" (or some other natural property), and "wrong" just is "causes suffering" in the way that "hot" just is "has high mean molecular kinetic energy." If this reduction succeeds, moral facts become as objective and discoverable as physical facts.
The central challenge to moral naturalism is G. E. Moore's open question argument. Moore observed that no matter what natural property N you propose as the reduction of "good," it always seems to be an open, intelligible question whether N things are actually good. "Pleasure promotes survival—but is survival good?" feels like a meaningful question in a way that "water contains hydrogen—but does H₂O contain hydrogen?" does not. Moore argued this shows "good" cannot be identical with any natural property; it names something sui generis. Naturalists have responded in several ways: some deny the intuition is reliable, others propose more sophisticated identifications (psychologically robust well-being rather than simple pleasure), and still others argue the "open question" appearance results from ignorance of the identity, just as ancient people could have wondered whether water was really H₂O.
A further complication comes from Hume's is-ought gap: you cannot derive what *ought* to be the case from what *is* the case. Moral naturalists must either argue that this constraint is overstated or explain how their natural-property identifications don't violate it. Many contemporary naturalists bite the bullet and argue that once the identity between moral properties and natural properties is established, the is-ought gap poses no special obstacle—the identity itself bridges the gap, just as the identity of heat with molecular motion lets you infer facts about temperature from facts about molecules. Whether this response succeeds is one of the central debates in contemporary metaethics.
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