Goodness and Moral Twin Earth


Terry Horgan and Mark Timmons’s “Moral Twin Earth” thought experiment allegedly undercuts virtually any form of naturalist moral realism. I argue that a neo-Aristotelian conception of moral properties defeats Moral Twin Earth. Developing themes in the work of Peter Geach, Philippa Foot, and Rosalind Hursthouse, I sketch an Aristotelian moral semantics that is unique in construing terms like ‘right’ and ‘good’ exclusively as attributive adjectives that denote relational properties. On this view, moral goodness is a relational property predicated of those human beings that satisfy kind-relative criteria of goodness; i.e., morally good human beings are good qua human being. The reference of moral terms is therefore fixed by the natural properties of human beings. This account ensures that moral facts cannot differ across worlds in which the relevant natural facts do not differ and thus defeats Moral Twin Earth.

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  1. 1.

    Horgan and Timmons (1991, 1992a, b)

  2. 2.

    Horgan and Timmons (2009)

  3. 3.

    Holland (2001).

  4. 4.

    Horgan and Timmons (2009, p. 221).

  5. 5.

    Moore (1988).

  6. 6.

    See, for example, Boyd (1988), Sturgeon (1985), Brink (1989).

  7. 7.

    Kripke (1980).

  8. 8.

    Putnam (1975).

  9. 9.

    Ibid., p. 223.

  10. 10.

    It’s possible that competent speakers on Earth associate the description “being composed of H2O” with the term “water.” Similarly, competent speakers on Twin Earth may associate the description “being composed of XYZ” with the term “water.” Thus, Putnam asks us to imagine both Earth and Twin Earth as they were in 1750, before the development of modern chemistry. In 1750, Earthlings and Twin Earthlings would not associate “water” with descriptions of its chemical composition. Rather they will associate similar, if not identical, descriptions with the term “water,” e.g., “the clear liquid that fills the oceans.” Putnam claims that even in 1750, the term “water” referred to H2O on Earth and XYZ on Twin Earth. Ibid,. p. 224.

  11. 11.

    Boyd (1988, p. 195). Boyd has since revised his semantics. For his latest account, see Boyd (2003).

  12. 12.

    Horgan and Timmons (1991, p. 458.)

  13. 13.

    Ibid,. p. 459.

  14. 14.


  15. 15.

    Ibid,. p. 460.

  16. 16.

    Timmons (1999, p. 62ff)

  17. 17.

    Horgan and Timmons (2000)

  18. 18.

    Geach (1967)

  19. 19.

    Ibid., p. 64.

  20. 20.

    Moore writes, “Ethics is undoubtedly concerned with the question what good conduct is; but, being concerned with this, it obviously does not start at the beginning, unless it is prepared to tell us what is good as well as what is conduct. For ‘good conduct’ is a complex notion: all conduct is not good; for some is certainly bad and some may be indifferent. And on the other hand, other things, besides conduct, may be good; and if they are so, then, ‘good’ denotes some property, that is common to them and conduct; and if we examine good conduct alone of all good things, then we shall be in danger of mistaking for this property, some property which is not shared by those other things.” Moore (1988, p. 2).

  21. 21.

    A similar analysis might hold for “right.” No property constitutes the right move in a chess match, the right program for installing an operating system, or the right bit for a given drill. See Thomson (2006).

  22. 22.

    Wright (1963).

  23. 23.

    Thomson (1997, p. 275). Thomson’s account differs from Geach’s in certain respects, but I omit discussion of such details because they are orthogonal to my purpose here.

  24. 24.

    Smith (1994).

  25. 25.

    Jackson (1998).

  26. 26.

    Horgan and Timmons (2009).

  27. 27.

    There are, of course, instances of the predicate ‘good α’ in which α’s are not functional concepts, and thus, the criterion of evaluation is something other than functionality.

  28. 28.

    For my purposes here, when ‘β’ is a functional concept, I will focus on the function of β, ignoring other semantic characteristics it might have. At least part of its semantic meaning is its functional role. There might be more to consider, e.g., part of the meaning of the term ‘guitar’ is given not only by its function, but also by its history. To simplify, I will simply focus on functions.

  29. 29.

    For a more detailed account, see Thomson (1997).

  30. 30.

    Foot (2003, p. 39).

  31. 31.

    See, for instance, Casebeer (2005), Foot (2003), Hursthouse (1999), Nussbaum (2000).

  32. 32.

    Foot (2003, p. 49), italics in the original.

  33. 33.

    Geach (1967, p. 72).

  34. 34.

    One might dispute that evaluations of states of affairs tout court are essential to utilitarianism or consequentialism broadly construed. For the claim that states of affairs are the locus of utilitarian evaluation, see Thomson (1994).

  35. 35.

    Hursthouse (1999, p. 195).

  36. 36.

    Ibid., p. 197.

  37. 37.

    It is possible that, as a contingent matter of fact, humans who pursue characteristic human ends maximize utility. However, in this case, the fact that good humans in the Aristotelian sense are also good humans in the utilitarian sense would be coincidental. On the utilitarian view, there is no necessary connection between being a good human and functioning in characteristically human ways. Similar remarks can be made about deontology. As a contingent matter of fact, characteristic human ends might align with Kantian moral duties. However this overlap would be purely coincidental.

  38. 38.

    This point can be made in another way. Suppose ethologists discover that the mythical chupacabra is a real species. Prior to empirical examination, these ethologists would know what it would mean for a chupacabra to be good, even though they have no idea which natural properties are the term’s referents.


  1. Boyd, R. (1988). How to be a moral realist. In G. Sayre-McCord (Ed.), Essays on moral realism (pp. 181–228). Ithaca: Cornell University Press.

    Google Scholar 

  2. Boyd, R. (2003). Finite beings, finite goods: The semantics, metaphysics, and ethics of naturalist consequentialism, part I. Philosophy and Phenomenological Research, 66, 505–553.

    Article  Google Scholar 

  3. Brink, D. (1989). Moral realism and the foundations of ethics. Cambridge: Cambridge University Press.

    Book  Google Scholar 

  4. Casebeer, W. (2005). Natural ethical facts: Evolution, connectionism, and moral cognition (p. 2003). Cambridge, MA: The MIT Press, Foot.

    Google Scholar 

  5. Foot, P. (2003). Natural goodness. Oxford: Clarendon Press.

    Google Scholar 

  6. Geach, P. (1967). Good and evil. In P. Foot (Ed.), Theories of ethics (pp. 64–73). Oxford: Oxford University Press.

    Google Scholar 

  7. Holland, S. (2001). Dispositional theories of value meet moral twin earth. American Philosophical Quarterly, 38, 177–195.

    Google Scholar 

  8. Horgan, T., & Timmons, M. (1991). New wave moral realism meets moral twin earth. Journal of Philosophical Research, 16, 447–465.

    Article  Google Scholar 

  9. Horgan, T., & Timmons, M. (1992a). Troubles on moral twin earth: Moral queerness revived. Synthese, 92, 221–260.

    Article  Google Scholar 

  10. Horgan, T., & Timmons, M. (1992b). Troubles for new wave moral semantics: The ‘Open Question Argument’ revived. Philosophical Papers, 21, 153–175.

    Article  Google Scholar 

  11. Horgan, T., & Timmons, M. (2000). Copping out on moral twin earth. Synthese, 124, 139–152.

    Article  Google Scholar 

  12. Horgan, T., & Timmons, M. (2009). Analytical moral functionalism meets moral twin earth. In I. Ravenscroft (Ed.), Essays on the philosophy of Frank Jackson (pp. 221–236). Oxford: Blackwell.

    Google Scholar 

  13. Hursthouse, R. (1999). On virtue ethics. Oxford: Clarendon Press.

    Google Scholar 

  14. Jackson, F. (1998). From metaphysics to ethics. Oxford: Clarendon University Press.

    Google Scholar 

  15. Kripke, S. (1980). Naming and necessity. Cambridge: Harvard University Press.

    Google Scholar 

  16. Moore, G. (1988). Principia ethica. Amherst, NY: Prometheus Books.

    Google Scholar 

  17. Nussbaum, M. (2000). Women and human development: The capabilities approach. Cambridge: Cambridge University Press.

    Book  Google Scholar 

  18. Putnam, H. (1975). The meaning of meaning. In H. Putnam (Ed.), Mind, language, and reality (pp. 215–271). Cambridge: Cambridge University Press.

    Chapter  Google Scholar 

  19. Smith, M. (1994). The moral problem. Oxford: Basil Blackwell.

    Google Scholar 

  20. Sturgeon, N. (1985). Moral explanations. In D. Copp & D. Zimmerman (Eds.), Morality, reason, and truth (pp. 49–78). Rowman and Littlefield: Totowa, NJ.

    Google Scholar 

  21. Thomson, J. (1994). Goodness and utilitarianism. Proceedings and Addresses of the American Philosophical Association, 67, 7–21.

  22. Thomson, J. (1997). The right and the good. Journal of Philosophy, 94, 273–298.

    Article  Google Scholar 

  23. Thomson, J. (2006). The legacy of principia. In T. Horgan & M. Timmons (Eds.), Metaethics after Moore (pp. 233–254). Oxford: Oxford University Press.

    Google Scholar 

  24. Timmons, M. (1999). Morality without foundations: A defense of ethical contextualism. New York: Oxford University Press.

    Google Scholar 

  25. Wright, G. (1963). The varieties of goodness. London: Routledge and K. Paul.

    Google Scholar 

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Thanks are due to Julia Annas, Nathan Ballantyne, Matt Bedke, Jason Brennan, Ian Evans, David Schmidtz, Frans Svensson, Mark Timmons, Kevin Vallier, and an anonymous referee for this journal.

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Correspondence to Christopher Freiman.

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Freiman, C. Goodness and Moral Twin Earth. Erkenn 79, 445–460 (2014).

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  • Noun Phrase
  • Natural Property
  • Moral Fact
  • Competent Speaker
  • Natural Kind Term