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Can One Be Two?

A Synopsis of the Twinning and Personhood Debate

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Abstract

It has been argued that the possibility of monozygotic twinning disproves the conservative position on foetal personhood that defines the foetus as a person from conception. This article will canvass arguments for and against this proposition, in order to arrive at a conclusion as to the relative strength of this finding.

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References

  1. Joseph F. Donceel, ‘Immediate Animation and Delayed Hominization,’ Theological Studies 31, 1970, 76–105; James Diamond, ‘Abortion, Animation and Biological Hominization,’ Theological Studies 36,1975, 30–24; Thomas A. Shannon & A. B. Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ Theological Studies 51, 1990, 603–626; Jason T. Eberl, ‘The Beginning of Personhood: A “Thomistic Biological Analysis’” Bioethics 14, 2000,134–157; Barry Smith and Berit Brogaard, ‘Sixteen Days,’ Journal of Medicine and Philosophy 28, 2003, 45–78; David W. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ Social Theory and Practice 31,2005, 51–75.

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  2. This is a simplistic use of the term Membrane/ and is purely designed to succinctly illustrate the point that the twins at this stage exist within the same limited physical parameters. One might more accurately label this membrane the zona pellucida, but as the pre-implantation embryo(s) ‘hatches’ from the zona pellucida at some point prior to implantation (whereby the zona pellucida degenerates), and a new type of membrane forms the outer layer of the pre-implantation embryo, it is in the interests of overall clarity to simply make the point that the biological processes under discussion occur within the same ‘membrane’ (broadly denned).

  3. The zygote may also develop into nothing more than a hydatidiform mole or teratoma. However, the implications of this phenomenon will not be pursued here. Furthermore, there is a strong argument that such organisms are not the result of‘normal, biologically complete, conceptions’ but are in fact ‘flawed or deficient fertilizations’ (Francis Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ Christian Bioethics 10, 2004, 51). Thus, such organisms cannot really be said to originate in zygotes, and therefore they are not appropriate targets for the individuation argument canvassed in this paper — see Stephen Heaney, ‘The Human Soul in the Early Embryo,’ The Thomist 56,1991,46; and Lisa Sowle Cahill, ‘The Embryo and the Fetus: New Moral Contexts,’ Theological Studies 54, 1993, 136.

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  4. It should be noted that, as the number of cells constituting the pre-implantation human organism increases, biology provides different labels. For instance, the original single celled fertilized egg is the ‘zygote‘; the approximately 16 celled organism is a ‘morula‘; and the approximately 32+ celled organism is a ‘blastocyst.’ It is only upon implantation that the term ‘embryo’ is generally utilised. However, for present purposes (and in the interests of overall clarity) this article will refer to the ‘pre-implantation embryo’ for all stages of the human organism from immediately post-zygote stage (i.e., immediately post a single celled organism) to implantation stage.

  5. See Anne McLaren, ‘Where to Draw the Line?’ Proceedings of the Royal Institution 56, 1984, 101–121; Louis M. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ Philosophy 81, 2006, 463; Smith & Brogaard, ‘Sixteen Days,’ 54–58.

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  6. Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 613.

  7. See John Mahoney, Bioethics and Belief, London: Sheed and Ward, 1984, 66–67; Thomas A. Shannon, ‘Fetal Status: Sources and Implications,’ Journal of Medicine and Philosophy 22, 1997, 420; Smith & Brogaard, ‘Sixteen Days,’ 55–56.

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  8. Diamond, ‘Abortion, Animation and Biological Hominization,’ 311-312; Marc Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ Utilitas 23, 2011, 189; John Burgess, ‘Could a Zygote Be a Human Being?’ Bioethics 24, 2010, 63.

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  9. For a more in depth discussion of the biological process of twinning see Smith & Brogaard, ‘Sixteen Days,’ 54–58; Eberl, ‘The Beginning of Personhood,’ 141–149; William Werpehowski, ‘Persons, Practices, and the Conception Argument,’ Journal of Medicine and Philosophy 22, 1997, 482–485.

  10. For the purposes of this article, I will ignore the fact that conception is a process, and not a moment, as advocates of this position appear to assume. For a discussion of this issue see Mark Rankin, “The Roman Catholic Church and the Foetus: A Tale of Fragility,’ Flinders Journal of Law Reform 10, 2007, 282; and Lynne Rudder Baker, ‘When Does a Person Begin?’ Social Philosophy and Policy 22, 2005, 26.

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  11. For further discussion on this view of immediate ensoulment see Mark Rankin, ‘The Roman Catholic Church and the Foetus,’ 275–281.

  12. There also exist ‘cross-over’ points of view that float between these two camps — see, e.g., Lee, who favours a secular conservative position that he labels the ‘substance ‘view of personhood, while simultaneously supporting the principle of immediate ensoulment — Patrick Lee, ‘A Christian Philosopher’s View of Recent Directions in the Abortion Debate,’ Christian Bioethics 10, 2004, 7–31; and Patrick Lee, Abortion and Unborn Human Life, Washington DC: Catholic University of America, 1996. Another example of this position might be the Catholic Church, which although clearly adopting the conservative theological view, nonetheless tends to buttress that view by reference to the secular science of genetics — see Pope Pius XII, ‘Humani Generis, Acta Apostolicae Sedis 42, 1950, 568, 574-575; and John Paul II, ‘The Gospel of Life: Evangelium Vitae,’ Acta Apostolicae Sedis 87, 1995, 401.

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  13. Christopher Tollefsen, ‘Embryos, Individuals, and Persons: An Argument Against Embryo Creation and Research,’ Journal of Applied Philosophy 18, 2001, 69; George Khushf, ‘Embryo Research: The Ethical Geography of the Debate,’ Journal of Medicine and Philosophy 22, 1997, 505; Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 612–613&623; Smith & Brogaard, ‘Sixteen Days,’ 45–46.

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  14. See, e.g., Sacred Congregation for the Doctrine of the Faith, ‘Declaration on Procured Abortion,’ Acta Apostolicae Sedis 66, 1974, 730; John Paul II, ‘The Gospel of Life,’ 401; Mahoney, Bioethics and Belief 62-67; Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 613-614; and Eberl, ‘The Beginning of Personhood,’ 137-151.

  15. This panel was formed by the US National Institutes of Health in January 1994. It provided a report in September 1994, and the Advisory Committee to the Director of the National Institutes of Health unanimously approved the report later that year.

  16. Carol A. Tauer, ‘Embryo Research and Public Policy; A Philosopher’s Appraisal,’ Journal of Medicine and Philosophy 22, 1997, 427–431.

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  17. See Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 191, who labels the same position ‘the twinning/fusion argument.’

  18. Indeed, the level of scholarship in this area has led Curtis to conclude that TA is the major argument against the conservative position on foetal personhood — Benjamin L. Curtis, ‘A Zygote Could be a Human: A Defence of Conceptionism Against Fission Arguments,’ Bioethics 26, 2012, 136.

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  19. Mark Johnson, ‘Quaestio Disputata: Delayed Hominization — A Rejoinder to Thomas Shannon,’ Theological Studies 58, 1997, 714.

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  20. Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ 47.

  21. Johnson suggests that twinning only occurs in 0.25% of cases, whereas Beckwith believes it to be in only 0.003% of cases — see, respectively, Mark Johnson, ‘Delayed Hominization: Reflections on Some Recent Catholic Claims for Delayed Hominization,’ Theological Studies 56, 1995, 751–754; Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ 47. There is more support for Johnson’s figure — see Gregor Damschen, Alfonso Gomez-Lobo, and Dieter Schonecker, ’sixteen days? A Reply to B. Smith and B. Brogaard on the Beginning of Human Individuals,’ Journal of Medicine and Philosophy 31, 2006, 172.

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  22. Burgess, ‘Could a Zygote Be a Human Being?’ 65.

  23. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 54-69; Donceel, ‘Immediate Animation and Delayed Hominization,’ 98-99; Diamond, ‘Abortion, Animation and Biological Hominization,’ 312.

  24. See, e.g., Diamond, ‘Abortion, Animation and Biological Hominization,’ 315–319.

  25. As stated earlier, there is some cross-over here with the conservative theological view as it is generally agreed that only individuals have souls. This point is well made by Christian Munthe, ‘Divisibility and the Moral Status of Embryos,’ Bioethics 15, 2001, 383–385. It also works in the other direction, as Ford, an early advocate of TA as against the theological conservative view, clearly demonstrated the applicability of TA to the secular conservative view — Norman Ford, When Did I Begin? Conception of the Human Individual in History, Philosophy and Science, Cambridge, UK: Cambridge University Press, 1988, 119-182. It should, however, be noted that Ford is no longer an advocate of TA, and now adopts a more conventional Catholic view on foetal personhood — see, e.g., Norman Ford, ‘The Moral Significance of the Human Foetus,’ in Richard E. Ashcroft, Angus Dawson, Heather Draper, and John R. McMillan (eds), Principles of Health Care Ethics, Hoboken NJ: Wiley, 2nd ed, 2007, 387

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  26. Of course, there is also no reason to assume that souls do not exist — see Michael J Selgelid, ‘Moral uncertainty and the moral status of early human life,’ Monash Bioethics Review 30, 2012, 54–55.

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  27. For a comprehensive metaphysical discussion of‘individuation’ — see Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 467-475.

  28. See Smith & Brogaard, ’sixteen Days,’ 60; Rudder Baker, ‘When Does a Person Begin?’ 26.

  29. Ford, When Did I Begin? 175.

  30. Smith & Brogaard, ’sixteen Days,’ 46.

  31. Ibid, 55. Indeed, they argue that some forms of yeast are ‘more properly unified’ -Ibid, 60.

  32. Ibid, 49.

  33. Ibid, 58.

  34. Ibid, 55.

  35. Ibid, 59.

  36. Ibid, 60.

  37. Ibid, 66.

  38. Rudder Baker, ‘When Does a Person Begin?’ 27; Smith & Brogaard, ’sixteen Days,’ 63. It is also of interest to note that the appearance of the primitive streak has been interpreted to signify the creation of a real human body in the hylomorphic sense -see Eberl, ‘The Beginning of Personhood,’ 137-151.

  39. Diamond, ‘Abortion, Animation and Biological Hominization,’ 316.

  40. Smith & Brogaard, ’sixteen Days,’ 60.

  41. Ibid, 62-63.

  42. Consequently, if twinning has occurred (and recombination has not), we would have had two pre-implantation embryos, that at this point may now be defined as two distinct individuals according to this view.

  43. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 497-499.

  44. Johnson, ‘Delayed Hominization,’ 744-749 & 763.

  45. Ibid, 753.

  46. Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ 49-50.

  47. Ibid, 49. Also see Tollefsen, ‘Embryos, Individuals, and Persons,’ 71-74; Lee, ‘A Christian Philosopher’s View of Recent Directions in the Abortion Debate,’ 9.

  48. Jan Deckers, ‘Why Eberl is Wrong: Reflections on the Beginning of Personhood,’ Bioethics 21, 2007, 274.

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  49. Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 196-197.

  50. Damschen et al., ’sixteen days? A Reply,’ 170.

  51. In this respect it is significant that ‘[t]here is not only a complete, connected external boundary, but, more precisely, a membrane or a physical covering — the zona pellucida — surrounding the cells. This membrane does not divide or disappear. The division takes place within its boundaries.’ Ibid, 169.

  52. As evidence of the fluid nature of such ‘facts,’ it is interesting to note that Eberl changed his view of biology, from holding that we are viewing a mere cluster of cells, to finding that there is ‘evidence of an inchoate organization and intercommunication among the cells that constitute an early embryo…functional interdependence among the cells.’ — Jason T. Eberl, ‘A Thomistic Perspective on the Beginning of Personhood: Redux,’ Bioethics 21, 2007, 284.

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  53. Tollefsen, ‘Embryos, Individuals, and Persons,’ 71-74.

  54. Khushf, ‘Embryo Research,’ 508.

  55. Ibid, 507-509; Tollefsen, ‘Embryos, Individuals, and Persons,’ 71-74; Jason Morris, ‘;Substance Ontology Cannot Determine the Moral Status of Embryos,’ Journal of Medicine and Philosophy 37, 2012, 349. Cf Thomas A. Shannon, ‘Response to Khushf,’ Journal of Medicine and Philosophy 22, 1997, 527.

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  56. Jeff McMahan, The Ethics of Killing: Problems at the Margins of Life, New York: Oxford University Press, 2002, 28–29.

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  57. Thomas A. Shannon, ‘Delayed Hominization: A Further Postscript to Mark Johnson,’ Theological Studies 58, 1997, 716.

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  58. In addition, it should be noted that there is an argument that biological unity or genetic distinctiveness is not the same as ontological individuality in any case — see Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 612-614.

  59. See, e.g., Smith & Brogaard, ’sixteen Days/ 66.

  60. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 464.

  61. Smith & Brogaard, ’sixteen Days/ 66.

  62. Ibid, 67.

  63. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 476-479; and Munthe, ‘Divisibility and the Moral Status of Embryos,’ 394-397.

  64. John Haldane and Patrick Lee, ‘Aquinas on Human Ensoulment, Abortion and the Value of Life,’ Philosophy 78, 2003, 273; Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ 47-48; Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 487.

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  65. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 479.

  66. In addition, the simple recognition that reproduction may occur via organism division is arguably another reason why the capacity for such division should not preclude an organism from being described as an individual — see Khushf, ‘Embryo Research,’ 505–509; A Chadwick Ray, ‘Humanity, Personhood, and Abortion,’ International Philosophical Quarterly 25, 1995, 244.

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  67. Smith & Brogaard, ’sixteen Days,’ 69.

  68. It might be disputed that our cells are not totipotent in such a scenario because cloning requires that the particular somatic cell be inserted into the enucleated egg cell and activated before it may be said to be totipotent. In response to this argument I say that: 1. The argument is predicated upon current technology, and my example is set in an undefined ‘future,’ in which it is feasible to assume that cloning may not require this level of external manipulation; and 2.1 define a ‘totipotent’ cell (for the purposes of this example) to mean a cell that is ‘capable of developing into another whole organism,’ and the fact that such external manipulation is required does not preclude a finding that this definition is met (this is a similar definition to that adopted by Ford — see Norman Ford, The Prenatal Person: Ethics from Conception to Birth, Oxford: Blackwell Publishing, 2002, 56).

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  69. It might be argued that an answer to this question depends upon whether genetic duplication results in identity duplication. I agree with Manninen that one does not necessarily result in the other — see Bertha Alvarez Manninen, ‘Cloning and individuality: Why Kass and Callahan are wrong (but maybe a little right),’ Monash Bioethics Review 30, 2012, 66–73. This issue of personal identity will not be discussed in this paper. For such an in-depth analysis see McMahan, The Ethics of Killing, 3-94.

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  70. Beckwith, ‘The Explanatory Power of the Substance View of Persons,’ 45-49. The cloning example is another way of illustrating Parfit’s split-brain thought experiment on this issue — see Derek Parfit, Reasons and Persons, Oxford: Oxford University Press, 1984, 254–264.

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  71. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 485.

  72. Smith & Brogaard, ’sixteen Days,’ 66; Lee, ‘A Christian Philosopher’s View of Recent Directions in the Abortion Debate,’ 10-12.

  73. It is also arguable that the focus on the ‘natural’ ability to divide places too much weight on ‘natural’ — see Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 194.

  74. Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 194.

  75. Ibid.

  76. Ramsey concludes on the basis of this alien illustration that an ability to twin or divide must be irrelevant to any findings as to a being’s moral standing — Ibid, 194-195.

  77. Munthe, ‘Divisibility and the Moral Status of Embryos,’ 387-388.

  78. Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 192. Munthe raises the related point that such reasoning is incompatible with the view that human persons possess unique values that have meaning — see Munthe, ‘Divisibility and the Moral Status of Embryos,’ 387-388. 79 Jean Porter, ‘Individuality, Personal Identity, and the Moral Status of the Pre-embryo: A Response to Mark Johnson,’ Theological Studies 56, 1995, 767.

  79. Damschen et al, ’sixteen days? A Reply,’ 172-173.

  80. Jeff McMahan, ‘Killing Embryos for Stem Cell Research,’ Metaphilosophy 38, 2007, 177; Deckers, ‘Why Eberl is Wrong: Reflections on the Beginning of Personhood,’ 275; Eberl, ‘A Thomistic Perspective on the Beginning of Personhood: Redux,’ 287; Deckers also believes that there is no good reason to hold that divisibility precludes ensoulment — see Deckers, ‘Why Eberl is Wrong: Reflections on the Beginning of Personhood,’ 278-280.

  81. Burgess, ‘Could a Zygote Be a Human Being?,’ 66.

  82. Eberl, ‘A Thomistic Perspective on the Beginning of Personhood: Redux,’ 285; Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 613.

  83. Diamond, ‘Abortion, Animation and Biological Hominization,’ 315.

  84. Ibid, 312 & 321; Donceel, ‘Immediate Animation and Delayed Hominization‘; Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo.‘

  85. Carol A Tauer, ‘The Tradition of Probabilism and the Moral Status of the Early Embryo,’ in Patricia Beattie Jung & Thomas A Shannon (eds), Abortion and Catholicism: The American Debate, Virginia: Crossroad, 1988, 56

  86. Eberl, ‘The Beginning of Personhood,’ 149-150; Mahoney, Bioethics and Belief, 62-67; Diamond, ‘Abortion, Animation and Biological Hominization,’ 321.

  87. Mahoney, Bioethics and Belief, 66-67.

  88. For example, Shoemaker suggests that there exist four possibilities when pre-implantation embryo A becomes twins Al and A2, but only two of those will be dealt with here, as the remaining two are not consistent with immediate animation — see Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 63-65.

  89. See Donald McCarthy, ‘Moral Responsibility for Abortion, Euthanasia, and Suicide,’ in Edward Gratsch (ed), Principles of Catholic Theology: A Synthesis of Dogma and Morals, New York: Alba, 1981, 333–337; Andrew C Varga, The Main Issues in Bioethics, New York: Paulist Press, 1984, 64-65.

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  90. Chadwick Ray, ‘Humanity, Personhood, and Abortion,’ 241-245; McMahan, ‘Killing Embryos for Stem Cell Research,’ 177; Damschen et al, ‘sixteen days? A Reply,’ 174; Munthe, ‘Divisibility and the Moral Status of Embryos,’ 390; Morris, ‘substance Ontology Cannot Determine the Moral Status of Embryos,’ 333.

  91. Eberl, ‘A Thomistic Perspective on the Beginning of Personhood: Redux,’ 285.

  92. See McMahan, The Ethics of Killing, 25–26; Ingmar Persson, ‘Two Claims About Potential Human Beings,’ Bioethics 17, 2003, 510; Burgess, ‘Could a Zygote Be a Human Being?,’ 62-64.

  93. Cf Howsepian who believes that there is ‘no insuperable metaphysical barrier to the possibility’ that B, but not C, is identical to A — see A. A. Howsepian, ‘Four Queries Concerning the Metaphysics of Early Human Embryogenesis,’ Journal of Medicine and Philosophy 33, 2008, 142. Also see Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 483.

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  94. Cessation is probably a better description because there is no corpse, so nothing appears to have ‘died’ in the ordinary sense of that word — see Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 190. That is, it is better to just hold that A goes out of existence or simply ‘ceases’ — see Burgess, ‘Could a Zygote Be a Human Being?,’ 69; McMahan, The Ethics of Killing, 27.

  95. Damschen et al, ‘sixteen days? A Reply,’ 173; and McMahan, The Ethics of Killing, 26.

  96. Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 193.

  97. Alternatively, we might suggest that only one of B and C souls is released and the other simply moves to inhabit the newly created D body — see Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 65.

  98. Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 199; and McMahan, The Ethics of Killing, 26.

  99. It is also inconsistent with the point made by Parfit that cell division should not be confused with death — see Parfit, Reasons and Persons, 262.

  100. An assumption most theologians support — see, e.g., Eberl, ‘A Thomistic Perspective on the Beginning of Personhood: Redux,’ 289.

  101. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 63-65.

  102. Cahill, ‘The Embryo and the Fetus: New Moral Contexts,’ 127; Daniel Dombrowski & Robert Deltete, A Brief, Liberal, Catholic Defense of Abortion, Champaign, IL: University of Illinois Press, 2000, 52; Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 618-619.

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  103. Diamond, ‘Abortion, Animation and Biological Hominization,’ 312-314.

  104. Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 618-619.

  105. Eberl, ‘The Beginning of Personhood,’ 156; Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 68.

  106. Burgess, ‘Could a Zygote Be a Human Being?,’ 64.

  107. Shannon & Wolter, ‘Reflections on the Moral Status of the Pre-embryo,’ 619.

  108. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 65.

  109. Deckers, ‘Why Eberl is Wrong: Reflections on the Beginning of Personhood,’ 281.

  110. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 69–75.

  111. Eugene Mills, ‘Dividing Without Reducing: Bodily Fission and Personal Identity,’ Mind 102, 1993, 37–51.

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  112. Rose Koch-Hershenov, ‘Totipotency, Twinning, and Ensoulment at Fertilization,’ Journal of Medicine and Philosophy 31, 2006, 155–160.

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  113. Deckers, ‘Why Eberl is Wrong: Reflections on the Beginning of Personhood,’ 274; Curtis, ‘A Zygote Could be a Human,’ 137–138.

  114. Koch-Hershenov, ‘Totipotency, Twinning, and Ensoulment at Fertilization,’ 160.

  115. Munthe, ‘Divisibility and the Moral Status of Embryos,’ 384.

  116. Curtis, ‘A Zygote Could be a Human,’ 136-142.

  117. Burgess, ‘Could a Zygote Be a Human Being?,’ 68.

  118. Curtis, ‘A Zygote Could be a Human,’ 136-142.

  119. Burgess, ‘Could a Zygote Be a Human Being?,’ 64-65.

  120. Ibid, 68.

  121. Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 63-65.

  122. Koch-Hershenov, ‘Totipotency, Twinning, and Ensoulment at Fertilization,’ 140.

  123. Ibid, 155.

  124. Ibid, 150–155.

  125. See, e.g., Shoemaker, ‘Embryos, Souls, and the Fourth Dimension,’ 66.

  126. Eberl, ‘A Thomistic Perspective on the Beginning of Personhood: Redux,’ 286.

  127. Munthe, ‘Divisibility and the Moral Status of Embryos,’ 384.

  128. Rudder Baker, ‘When Does a Person Begin?,’ 25-27.

  129. Guenin, ‘The Nonindividuation Argument Against Zygotic Personhood,’ 485. Note: Guenin makes the argument from the other side — namely, that the potential of each cell (prior to the loss of totipotency) to become a distinct human person is not the same as each cell being a distinct human person simply by virtue of this potentiality -but the premise is similar, if not identical.

  130. Burgess, ‘Could a Zygote Be a Human Being?’ 62-64.

  131. The same reasoning would apply to recombination: such that the adult human person could only trace themselves to D, and not to B and C that recombined to form D — see Howsepian, ‘Four Queries Concerning the Metaphysics of Early Human Embryogenesis,’ 143-46.

  132. This conclusion is further substantiated when one takes into consideration the placenta: that is, a zygote gives rise to a human being and a placenta, and it cannot be both, so the human being can only trace herself to that point in embryonic development and not before — see Burgess, ‘Could a Zygote Be a Human Being?,’ 61-70.

  133. Ramsey, ‘Twinning and Fusion as Arguments against the Moral Standing of the Early Human Embryo,’ 202.

  134. Koch-Hershenov, Totipotency, Twinning, and Ensoulment at Fertilization.‘

  135. Curtis, ‘A Zygote Could be a Human,’ 142; Howsepian, ‘Four Queries Concerning the Metaphysics of Early Human Embryogenesis,’ 154.

  136. Curtis, ‘A Zygote Could be a Human,’ 137.

  137. Ibid.

  138. Ibid. Cf St John concerning the issue of how much change is required before identity may be said to be violated — see Jeremy St John, ‘And on the fourteenth day…potential and identity in embryological development,’ Monash Bioethics Review 27, 2008, 16–18.

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  139. Deckers, ‘Why Eberl is Wrong: Reflections on the Beginning of Personhood,’ 279.

  140. Morris, ‘substance Ontology Cannot Determine the Moral Status of Embryos,’ 348.

  141. Ibid, 333.

  142. Ibid, 349.

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    Mark Rankin

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Rankin, M. Can One Be Two?. Monash Bioethics Review 31, 37–59 (2013). https://doi.org/10.1007/BF03351547

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