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What challenge does the existence of moral facts pose for moral realism?

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The Greek philosopher Plato — Realism Plato, marble portrait bust copied from an original of the 4th century bce; in the Capitoline Museums, Rome.

The existence of moral facts poses significant challenges for moral realism, primarily on metaphysical and epistemological fronts.1

Metaphysical Challenges

Critics question whether a realm of distinctively moral facts can exist at all. These facts are often viewed as irreducible to, and inexplicable by, the facts of nature. Some philosophers, such as J.L. Mackie, argue that moral properties possess an intrinsic "queerness" compared to natural properties, leading them to reject the existence of such facts entirely in favor of an error theory.1

Epistemological Challenges

Even if moral facts existed, moral realism struggles to explain how humans could access them. Unlike logic or mathematics, which can be accessed through reason, morality does not appear to be discoverable through reason alone. Furthermore, there is no recognized special faculty—such as "moral intuition"—that reliably detects these truths, as such concepts are often seen as merely renaming the problem rather than solving it.1