Horgan and Timmons (
1992) suggest the following argument against metaethical naturalism: First, note that the naturalist is committed to there being some semantic story about how the reference of our moral terms gets fixed. For example, perhaps ‘right’ and ‘wrong’ refer to those natural properties of actions that causally regulate our practices of praise and blame. So, if the consequentialist property of
maximizing happiness is what causally regulates our praising practices, then ‘right’ will refer to the natural property of an act’s being happiness-maximizing.
Second, Horgan and Timmons point out that we can imagine a "Moral Twin Earth" -- a society very similar to ours but where the features identified in the naturalist's moral semantics play out slightly differently, such that they end up picking out a different natural property. So, in our above example, we might imagine a world much like ours except that the deontological property of
conforming to the categorical imperative is what causally regulates our counterparts’ practices of praise and blame. So, in that world, ‘right’ will refer to the property of conforming to the categorical imperative.
This naturalist theory then implies that we are talking past each other -- both speaking the truth in our own moral language -- when we affirm consequentialism and our Moral Twin Earth counterparts affirm Kantianism. This seems an unacceptable relativistic result, and violates our semantic intuition that the two parties are -– despite their different answers -– addressing the same moral question. Intuitively, we are
disagreeing with our Kantian counterparts, not merely speaking past one another. (Contrast the standard case of water/H2O: In regular Twin Earth, we have no semantic intuition that speakers genuinely disagree when we say “water is H2O” and our twins say “water is not H2O”. The standard Kripke/Putnam intuition is that the two parties are talking about different substances. This difference strongly suggests that it would be a mistake to model our moral semantics on the semantics of natural kind terms.)
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