Moral Twin Earth and the Problem with Reductive Naturalism

Palavi Chander

Moral Twin Earth and the Problem with Reductive Naturalism

Author(s)	Ms. Palavi Chander
Country	India
Abstract	The Moral Twin Earth argument, developed by Terence Horgan and Mark Timmons (1991), poses a notable challenge to reductive naturalism in metaethics. It imagines a hypothetical world closely resembling our own, where inhabitants use moral terms such as ‘good’ and ‘right’. However, on Twin Earth, these terms are tied to different natural properties than on Earth. For instance, while Earthlings might define “good” in terms of promoting happiness (a consequentialist perspective), Twin Earthlings might associate “good” with following universal moral rules (a deontological perspective). This raises important questions about how moral language works, particularly its meaning and reference. Suppose moral terms can point to different natural properties in other contexts, as the Moral Twin Earth argument suggests. In that case, this challenges the core claim of reductive naturalism; that moral properties are fixed and directly tied to specific natural properties. Instead of offering a universal foundation for moral language, reductive naturalism risks leading to semantic relativism, where moral discourse becomes contextual. Ultimately, this complicates the idea that moral facts can be fully reduced to natural facts. The Moral Twin Earth argument, based on Hilary Putnam’s (1973) Twin Earth thought experiment, highlights this complexity of moral discourse and casts doubt on the feasibility of reductive approaches within moral realism.
Keywords	Hilary Putnam, Horgan and Timmons, Moral Twin Earth (MTE), reductive naturalism, Twin Earth
Published In	Volume 8, Issue 4, July-August 2026
Published On	2026-07-21

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Moral Twin Earth and the Problem with Reductive Naturalism

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