¹ Recently researchers have had some success at removing cells without destroying the blastocyst. It is possible that this procedure, if perfected, will end much of the controversy surrounding embryonic stem cell research. Other compromises may be found. It is possible that suitable stem cell lines can be generated using genetically non-viable embryos or embryos produced from parthenogenesis. The question of whether stem lines generated from these sources will prove as useful as those generated from developmentally normal embryos remains open. For the foreseeable future, embryonic stem cell research will continue to involve stem cell lines created through the destruction of developmentally normal embryos. See J. F. Battey Jr., L. K. Cole and C. A. Goldthwaite, ‘Regenerative Medicine’, National Institutes of Health (2006), ch. 8 <http://stemcells.nih.gov/info/2006report/2006Chapter8.htm>.

² I will not consider the difficulties that twinning and fusion pose for the religious concept of ensoulment. Most religious proponents of MSEHE no doubt believe that the EHE is also ensouled and that ensoulment plays an important role in a comprehensive account of the EHE's claim to strong moral standing. However, I will assume that, for the purposes of moral and political argument with non-religious persons, proponents of MSEHE intend to base their argument on what they regard as morally relevant natural properties, the EHE's totipotency and inner-directedness.

³ Sumner, L. W., Abortion and Moral Theory (Princeton, 1981), pp. 124–33CrossRefGoogle Scholar.

⁴ Tooley, Michael, ‘In Defense of Abortion and Infanticide’, The Abortion Controversy, 2nd edn., ed. Pojman, L. P. and Beckwith, F. J. (Belmont, CA, 1998), pp. 209–33Google Scholar; Tooley, Michael, Abortion and Infanticide (Oxford, 1983), pt. IIIGoogle Scholar; Warren, Mary Anne, ‘On the Moral and Legal Status of Abortion’, Ethics in Practice, 2nd edn., ed. LaFollette, H. (Malden, MA, 2002), pp. 72–82Google Scholar.

⁵ Sumner, Abortion, pp. 133–46. Of course, appeals to sentience do not, by themselves, make good sense of the fact that most of us afford higher moral standing to developmentally normal young human children than we do to adult cats and dogs. For a possible resolution to this problem, see Harman, Elizabeth, ‘The Potentiality Problem’, Philosophical Studies 114 (2003), pp. 173–98CrossRefGoogle Scholar. In any case, sentience depends on significant forebrain development, so there can be no real question whether the EHE possesses sentience. Because the EHE lacks sentience, it lacks even the most primitive forms of preferences and desires; so one may argue that it has no morally relevant interests, and, therefore, lacks even weak moral standing.

⁶ Marquis, Don, ‘Why Abortion is Immoral’, The Journal of Philosophy 86 (1989), pp. 183–202CrossRefGoogle ScholarPubMed.

⁷ Marquis, ‘Abortion’, pp. 201–2.

⁸ Marquis, Don, ‘The Moral Principle Objection to Human Embryonic Stem Cell Research’, Metaphilosophy 38 (2007), pp. 190–206CrossRefGoogle Scholar.

⁹ For example, McMahan, Jeff, The Ethics of Killing: Problems at the Margins of Life (Oxford, 2002), pp. 270–5CrossRefGoogle Scholar.

¹⁰ For what it is worth, I think that most opponents of embryonic stem cell research would prefer to say that the embryo is already a person in some important sense. That is to say that from the time of conception it already possesses strong moral standing.

¹¹ Finnis, John, ‘Some Fundamental Evils in Generating Human Embryos by Cloning’, Ethics and Law in Biological Research, ed. Mazzoni, C. M. (The Hague, 2002), pp. 99–106 (103)Google Scholar.

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¹³ Green, Ronald M., The Human Embryo Research Debates: Bioethics in the Vortex of Controversy (Oxford, 2001), pp. 142–4Google Scholar. Devolder, Katrien and Harris, John, ‘The Ambiguity of the Embryo: Ethical Inconsistency in the Human Embryonic Stem Cell Debate’, Metaphilosophy 38 (2007), pp. 153–69 (158)CrossRefGoogle Scholar.

¹⁴ Copland, Paul and Gillett, Grant, ‘The Biomedical Structure of a Human Being’, Journal of Applied Philosophy 20 (2003), pp. 123–31 (126)CrossRefGoogle Scholar.

¹⁵ Devolder and Harris, ‘Ambiguity’, p. 161.

¹⁶ Green, Ronald M., ‘Part III-Determining Moral Status’, American Journal of Bioethics 2 (2002), pp. 20–30 (26)CrossRefGoogle ScholarPubMed; Devolder and Harris, ‘Ambiguity’, pp. 163–6.

¹⁷ Chan, A. W. S., Dominko, T., Luetjens, C. M., Neuber, E., Martinovich, C., Hewitson, L., Simerly, C. R. and Schatten, G. P., ‘Clonal Propagation of Primate Offspring by Embryo-splitting’, Science 287 (2000), pp. 317–19CrossRefGoogle ScholarPubMed.

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¹⁹ Yu, Neng, Kruskall, Margot S., Yunis, Juan J., Knoll, Joan H. M., Uhl, Lynne., Alosco, Sharon, Ohashi, Marina, Clavijo, Olga, Husain, Zaheed, Yunis, Emilio J., Yunis, Jorge J., and Yunis, Edmond J., ‘Disputed Maternity Leading to Identification of Tetragametic Chimerism’, The New England Journal of Medicine 346 (2002), pp. 1545–52 (1545)CrossRefGoogle ScholarPubMed.

²⁰ Yu, et al., ‘Disputed Maternity’, p. 1545.

²¹ Kuhse, Helga and Singer, Peter, ‘Individuals, Humans and Persons: The Issue of Moral Status’, Embryo Experimentation, ed. Singer, P., Kuhse, H., Buckle, S., Dawson, K., and Kasimba, I. (Cambridge, 1990), pp. 65–75 (67)CrossRefGoogle Scholar.

²² Devolder and Harris, ‘Ambiguity’, p. 153.

²³ Finnis, ‘Fundamental Evils’, p. 102.

²⁴ McMahan, Jeff, ‘Killing Embryos for Stem Cell Research’, Metaphilosophy 38 (2007), pp. 170–89 (177)CrossRefGoogle Scholar.

²⁵ This argument should be considered against the background of the other arguments against MSEHE. Antagonists of MSEHE most likely endorse something along the lines of sentience as a prerequisite for any degree of moral standing. Their interlocutors, proponents of MSEHE, reject the possession of sentience as a necessary condition for moral standing, and these persons are likely to endorse something along the lines of Finnis's totipotence and inner-directedness criteria as sufficient to establish the EHE's strong moral standing. Now perhaps these criteria are based on a conflation of actual and potential characteristics. Likewise, these criteria may be unstable, threatening to extend moral standing to entities such as stem cells and somatic cells within human bodies. But these are other arguments. If the simple twinning/fusion argument is to provide a decisive refutation of MSEHE, it must avoid appeals to issues already taken up in the other arguments.

²⁶ Oderberg, David S., ‘Modal Properties, Moral Status, and Identity’, Philosophy and Public Affairs 26 (1997), pp. 259–98 (276)CrossRefGoogle Scholar.

²⁷ Parfit, Derek, Reasons and Persons (Oxford, 1984), pp. 254–5Google Scholar.

²⁸ As he notes, we may never develop the technology to establish the staggering number of connections between brain and body that would be needed to make each half of the brain functional in its new body. Likewise, we would face serious problems with respect to the division and maintenance of the lower brain. But such a division is possible, at least in principle. See Parfit, Reasons, p. 255.

²⁹ Oderberg, ‘Modal Properties’, pp. 277–8.

³⁰ Green, Human Embryo, pp. 142–4.

³¹ A spin on Derek Parfit's fission cases inspired by the science fiction television show Farscape. See Matt Ford (writer) and Ian Watson (director), ‘Eat Me’ (television series episode), Anthony Winley (producer), Farscape (Jim Henson Productions, 2001).

³² Of course, this story is even more far-fetched than Parfit's split-brain experiment stories, but this is not terribly important. What matters is whether we can conceive of such cases and whether we have clear intuitions about them. See Parfit, Reasons, p. 255.

³³ Oderberg, ‘Modal Properties’, pp. 275–8.

³⁴ A variation on what Kai Nielsen refers to as a well-known example (among philosophers) in his defence of act-utilitarianism. In the standard example, the fat man is facing outward and will survive if nothing is done to save the other persons trapped within the cave. See Nielsen, Kai, ‘Against Moral Conservatism’, Ethics 82 (1972), pp. 219–31 (222)CrossRefGoogle Scholar.

³⁵ Of course there is a good chance that many actual proponents of MSEHE would reject this view. Many religious opponents of embryonic stem cell research might endorse not only the strong moral standing of the EHE but also a very rigorous deontological view that forbids the killing of innocents under any circumstances.

³⁶ McMahan, ‘Killing Embryos’, p. 178.

³⁷ Marquis, ‘Moral Principle’, p. 200.

³⁸ Marquis, ‘Moral Principle’, p. 201.

³⁹ Marquis, ‘Moral Principle’, p. 201.

⁴⁰ George, Robert P. and Gómez-Lobo, Alfonso, ‘The Moral Status of the Human Embryo’, Perspectives in Biology and Medicine 48 (2005), pp. 201–10 (207)CrossRefGoogle ScholarPubMed.

⁴¹ George and Gómez-Lobo, ‘Moral Status’, p. 206.

⁴² George and Gómez-Lobo, ‘Moral Status’, p. 207.

⁴³ McMahan, ‘Killing Embryos’, p. 180.

⁴⁴ McMahan, ‘Killing Embryos’, p. 180.

⁴⁵ McMahan, Ethics, p. 26.

⁴⁶ Oberberg, ‘Modal Properties’, p. 270.

⁴⁷ McMahan, Ethics, p. 27.

⁴⁸ Harman, Elizabeth, ‘How Stem Cell Research Differs From Abortion’, Metaphilosophy 38 (2007), pp. 207–25 (215)CrossRefGoogle Scholar.

⁴⁹ Parfit thinks that psychological continuity matters in survival and identity, so how can his theory be used to support the moral standing of EHEs, non-sentient entities? As I explain in the Appendix, I am not using Parfit to establish that persons are, in a morally important sense, identical with their EHEs. Whatever it is that connects us to our EHEs, it cannot be psychological continuity. Nor am I engaged in defending the view that there is such a morally relevant connection. I am simply using a play on Parfit's argument to show that proponents of MSEHE need not regard twinning as the death of the originating embryo. See Parfit, Reasons, pp. 260–1.

⁵⁰ Parfit, Reasons, pp. 219–43.

⁵¹ Parfit, Reasons, pp. 234–43.

⁵² Parfit, Reasons, p. 256.

⁵³ Parfit, Reasons, pp. 261–5.

⁵⁴ Opponents of MSEHE may complain that it is inappropriate to use Parfit to defend a position that, at least typically, is endorsed by people who believe in souls. After all, Parfit's discussion of identity is intended to convince us of a reductionist view of identity. In other words, there is nothing more to identity and survival than psychological (and perhaps physical) continuity; there is nothing more than this, and there is no soul. In their comprehensive moral views, such persons may well have a difficult time wedding the notion of a soul to this defence of MSEHE, and I am fairly confident that any such attempt will appear ad hoc or ‘tacked on’ from the perspective of those who reject the concept. Here I think it is best to say that as long as proponents of MSEHE do not appeal to souls in their arguments for this position, it is best to leave this matter alone.

⁵⁵ The appeal to perspective here is, of course, somewhat metaphorical. See Harman, ‘Stem Cell Research’, p. 212.

⁵⁶ Harman, ‘Stem Cell Research’, p. 215.

⁵⁷ An early version of this article was presented at the meeting of the CS-IVR in Ottawa at Carleton University, Ontario, Canada on May 24, 2009. I thank the participants for their comments. I would also like to thank Samantha Brennan, Stephen Maitzen, Karen Myers, and Mark Vopat for their comments on previous versions of this article.