I. Twinning and Abortion
I consider abortion wrong because it is, I judge, the taking of a human life. Like any other taking of a human life, it wouldn’t necessarily be wrong always, but there should be a strong presumption against it.1
Why does it seem to me that abortion is the taking of a human life? First, because the entity in the final month of pregnancy seems other in no meaningful sense than the entity in the first hours and days after birth. Using one’s lungs to take in air is merely a developmental difference, not something that turns one from a nonhuman being into a human being. If the born child is human, so is the child a few weeks prior to birth.
Grasping that point, I then move backwards. There seems to be no difference between a not-yet-born human in the seventh month and one in the eighth month, apart from developmental differences analogous to breathing air. It is the same being, and if the fetus is human at eight months, it is human at seven.
And so it is at six.
At five. At four. At three. At two. At one.
Through all those months, the development of the unborn entity is continuous. The vast changes from early pregnancy to late are obvious; just compare a five-ounce fetus to one at five pounds. But similarly after birth: A five-year-old looks quite different from the same person at age thirty. Yet from day to day the changes (barring outside events) are so small as to go unnoticed. The human organism develops continuously, and goes on to nourish and repair itself continuously, while it does particular things at particular times.
I push this argument back as far as I can, and it seems to me I must say that at the moment of conception, or perhaps more precisely at the event of fusion of sperm cell with oocyte, the new human life begins.
Yet here, hesitation sets in. Is it reasonable to think that the zygote, the single-cell organism that results from sperm-oocyte fusion, is a human being? It contradicts instinct and habit to speak that way. One understands why an adult might object, “Surely I’m of more value than a single cell!” (meaning that it is incomprehensible to think of a zygote as a human being, and thus we do not have to treat zygotes in the ways we treat humans). One understands . . . and one thinks also about twinning.
We who care about unborn human life do not want to claim more than it is reasonable to say. Falsehoods, even in a good cause, remain false, and in the long run they will damage the good they try to uphold. Although my logic seems good to me, I wonder if it leads me to a true conclusion. Or does it take me too far? Is the sometime occurrence of twinning an indication that my logic indeed has taken me too far? Perhaps one should hold off speaking of a human individual until the embryo has developed past the point at which twinning is a possibility.
The twinning phenomenon sets forth a prima facie problem for the claim that it is at the moment of fusion of sperm and egg—the beginning of the embryo— that the life of a unique human being begins.
Gilbert Meilaender, for instance—a Lutheran ethicist and, in the day, member of the President’s Council on Bioethics (the council chaired by Leon Kass)— wrote in the first edition of his Bioethics: A Primer for Christians that we have “some good reasons to hesitate” about “identifying conception or fertilization as the point at which a new individual human being comes into existence.” The first of his reasons was that perhaps as many as half of fertilized ova fail to implant; on the assumption of life beginning at fertilization, that would mean that about half the human race “dies after a life of four to five days.” Second, until the possibility of twinning has passed, “the individuality of the developing entity is not firmly established.”2 In 1996, Meilaender did not advance these as conclusive arguments but merely reasons for epistemic caution.
Notably, Meilaender has dropped these “reasons to hesitate.”
Bioethics: A Primer for Christians has become a perennially popular introduction to thinking Christianly in this field; I myself have used it with various classes over the years and expect to use it again. (Its fourth edition is in press as I write this.) Each revision has aimed to bring the science up to date and to address emerging topics (such as embryonic research and conscience protections). With regard to the beginning of life, Meilaender soon abandoned his initial hesitation. He affirms: “After fertilization it is hard to find any other equally decisive break in the process of development.” He has dropped his concern about failures of early embryos to implant.3 With regard to the possibility of twinning, he says this “argument seems less persuasive to me than it once did—in part because its philosophical ground is doubtful, and in part because its basis in our knowledge of embryological development has become increasingly shaky.”4
What, then, is known about “embryological development” that would make twinning no objection to a claim that we have a human individual from the fusion that creates the initial zygote?
II. The Human Beginning
In her new book Untangling Twinning, Maureen Condic, a neurobiologist at the University of Utah, lays out the science with clarity and philosophical competence in a concise hundred pages (with another seventy-five pages of notes and bibliography).5 The science is crisply presented and graspable by a non-scientist who has done little reading in the field since college (e.g., yours truly). And her philosophical arguments are also clear and graspable, even if in turn they raise some questions about whether human beings have souls.
First she lays out some of the things that happen at the fusion of sperm and oocyte. The membranes of the cells “fuse, creating a single hybrid cell: the zygote or one-cell embryo.” (A zygote is just a name for the embryo at the one-cell stage.) This occurs “in less than a second.” The zygote “contains all the components of both sperm and egg,” which gives it “a unique molecular composition” different from either of them. “Within seconds, the zygote initiates a molecular cascade that will, over the next thirty minutes, result in chemical modifications that prevent additional sperm from binding to the cell-surface.” The zygote is clearly a new cell type, with composition and function that are different from either sperm or oocyte. Incidentally, she notes, this means that it is not scientific to speak of a “fertilized egg”: the zygote is not an egg.6
We are justified in speaking of the zygote as “a new human individual,” that is to say, a human organism rather than at best something which is or could be a part of a human organism. The “distinguishing feature” of an organism is “the interaction of parts in support of an integrated whole.” For a human organism, this interaction of parts in support of an integrated whole is manifested in four ways. First is development, the production of “a characteristic sequence of events that robustly results in the generation of a species-specific mature state.” Development, she underlines, “is the defining characteristic of all embryos from the zygote stage onward.” Second, there is “the ability to repair injury to restore the health and function of the entity as a whole.” This, embryos do with remarkable success. Third is “adaptation to changing environmental circumstances” in order to preserve “the health and overall function of the organism.” Embryos have been observed to have this facility also. And fourth is to show, at every stage of life, the “integrated function of parts to promote the health of the organism as a whole.” There is that “specific molecular cascade” that the zygote initiates “to direct its subsequent maturation.” At the four-cell stage, “individual blastomeres [cells] have distinct patterns of gene expression, different cellular function, and unique developmental capabilities,” meaning that the embryo’s functioning as a whole “reflects collaboration of its parts to generate a normal developmental sequence.” In contrast to tumors and the like, “only embryos establish a global pattern of interaction that benefits the entity as a whole,” a pattern that involves “dozens of distinct, globally integrated events,” initiated “within the first minutes and days of life,” “that are critical for its survival and healthy maturation.”7
It amazes me to learn, even in these rough layperson terms, of the immediate and complex processes that begin when sperm and oocyte unite. Before she turns to twinning, Condic wants the reader to understand that the zygote is a human organism at the beginning of that organism’s life-development, with the ability to repair itself and adapt to its environment, integrating its increasingly complex parts and functions to advance the health of the human organism as a whole.
I began with a negative reason for tracing the onset of human life to the fusion of the sperm and egg: the lack of any moment along the way at which we might say a non-human turned into a human. Condic gives a positive reason: The zygote is a new type of human cell, one that is the beginning of a new human organism. Nonetheless, what are we to make of twinning?
III. The Science of Twinning
The twinning we are concerned with is “monozygotic twinning”: an entity that begins as a single zygote but at some point becomes two entities, two embryos. Commonly speaking, these are called “identical” twins, in contrast to “fraternal” twins who began as two separate zygotes. What does science know about how and when so-called identical twins come about?
The general view is that it could happen at three possible embryonic stages. The earliest, which accounts for about 33 percent of these twins, comes prior to the blastocyst forming. Each part of the initial embryo, after the split, goes on to do what embryos do, forming itself as a blastocyst and so forth. About 66 percent of monozygotic twins form a bit later, by a splitting within the nowformed blastocyst but prior to its formation of the amniotic cavity. And a few such twins, less than one percent, are formed still later: These twins will share the amniotic cavity and in some cases will be conjoined.
Condic argues that in the latter two cases, twinning raises no question about the initial embryo. We already had a blastocyst, an established individual manifestly involved in the complex “unified developmental process” of developing as a human being; at some point, that human individual asexually reproduced. Asexual reproduction itself need not raise questions of the individuality of the original. We know of non-human organisms that can split or be split, with each part becoming a distinct individual; yet we don’t doubt that the original plant or worm (say) was truly a plant or worm before it became two plants or worms. Which is not to deny that there remain questions here. “Does the original embryo die or continue as one of the twins? What is the moral worth of the embryo prior to twinning? Who are the parents of the twins?”8 These questions Condic takes up later, but they do not per se raise questions about “the ontological status” of the zygote.
Twinning at the earliest stage, however, poses the harder question. Prior to the blastocyst stage, each cell of the embryo would be totipotent if it existed on its own, and any group of such cells would be totipotent.9 That is to say, in those very few days a complex, chemical organizational process is going on, but the cells involved are not irreversibly committed to that process. Thus the question: Did we have an individual before that pre-blastocyst division took place—or did we have merely a collection of cells capable of becoming one or more individuals? If, to take the earliest possible case, this twinning were to occur when the embryo was only two cells, does that not raise a question of whether the zygote, the initial single-celled embryo, was truly an individual being?
Condic marshals arguments, both positive and negative, to address this. First, such early twinning is highly unlikely in nature, if indeed it ever occurs. To be sure, in experiments twins can be “produced by splitting at the two-cell stage.” Yet even in the laboratory environment of ART (assisted reproductive technology, such as is used in IVF), “twinning in the first three days of life has never been reported.” She quotes from a 2013 article in the official journal of the International Society of Twin Studies: “We have never observed an embryo spontaneously splitting in half before the blastocyst stage in over thirty years of laboratory experience.”
Second, there are also positive evidences—she gives four of them—that “cells of the early embryo are both highly adherent to each other and strongly inclined to act as an integrated whole.” If such an early embryo were to split, there is “ample evidence” that “the twins would rapidly reanneal to form a single embryo.”10
What then is going on in these 33 percent of twinning cases? Condic offers an alternative explanation: that these twins (and perhaps also at least some of those slightly later 66 percent of cases) “arise by blastocyst splitting at hatching.” Hatching is the process whereby the embryo escapes from the zona pellucida in order to be able to implant. The zona protects the embryo as it travels down the fallopian tube. But to implant in the uterus, the outer cells of the embryo must interact with the uterine lining. Hence at the appropriate time these outer cells “secrete enzymes that degrade the zona, weakening it so that the embryo is able to squeeze out of a small hole.” In that process, the embryo’s cells may split apart as they move on to implant. Although we lack the tools to be able to observe this process in natural conception, “blastocyst splitting at hatching . . . has been observed in ART procedures.” Twinning at hatching—being as it were a natural but external hazard brought upon an embryo at this moment in its life11— would not raise a question about the embryo’s existence as a living, unified being that, from its origin as a zygote, was organized to develop as a human individual. Condic summarizes:
There is clear scientific evidence that the one-cell embryo or zygote initiates a developmental trajectory; that is, the zygote is manifestly a human organism. Therefore, twinning at the two-cell stage or later does not call into question the ontological status of the original embryo as a complete and individual human being.12
This answers the question of the individuality of the embryo prior to twinning, but it does not tell us how to think of the twins. Does the original human individual die and two new individuals take its place? Or, alternatively, does the original individual continue to exist as one of the twins, with the other twin being a new human whose life began at twinning?
Condic gives scientific evidence to support the latter interpretation. If (say, in an experiment) “a blastocyst-stage embryo is split and one half is discarded,” the remaining half quickly reorganizes itself. It reseals and resumes development; if its cells at that point are disproportionately one kind or another, then others are proliferated as needed, and so forth. In short, the remaining half works as an individual human organism and “proceeds as a unified whole along the developmental trajectory that was established by the zygote.” This process, she says, “most closely resembles” the biological process observed in wound healing.13
Now if the other half of the embryo, instead of being discarded, survives and regenerates its own “missing parts,” this would not change our view of the original embryo. Thus, although we may be unable to know (in some or all cases, or with certainty) “which half is the original embryo,” nonetheless “there is clearly no evidence that the original embryo has ceased to be.”14
Should we then say that the parents of one of the twins are the biological parents, but the “parent” of the other twin is the first twin, the original embryo? This is an important question philosophically, even if we are in ignorance about which twin is which.
Condic argues that the biological parents are parents of both, but in distinguishable senses. First, although something may be unfamiliar to us and unusual, that does not make it unnatural. We normally think of human reproduction as sexual, coming from the union of sperm and ovum. But asexual reproduction, namely twinning, “is a rare, yet completely natural, form of human generation.” Further, this rare asexual act remains part of the initial reproductive intention of the parents. “[R]eproductive acts are inherently ordered towards reproduction,” Condic writes. “The terms ‘mother’ and ‘father’ refer to those agents who participate in a sexual reproductive act as reproductive,” which such an act is—whether undertaken through instinct or conscious intention, through love, through pleasure, or whatever. The participants in a reproductive act are rightly called parents. By contrast, “when an embryo secretes enzymes to degrade” the protective covering so that it can implant, “this act is naturally ordered towards implantation, not reproduction.” If that hatching process accidentally, in some cases, brings about a twin, then in such a case an embryo reproduces asexually but not through an act ordered towards reproduction. The reproductive intention remains with the parents, who are the parents of each of the twins.15
In general terms, in any case where an embryo asexually reproduces and brings about a twin, that result is a natural but unusual consequence of an action whose intention lies elsewhere (e.g., implantation). So the original embryo does not die, and the parents of both embryos are the same.
IV. The Human Substantial Form (aka the Soul)
When we see that the zygote is a human organism, we are recognizing the presence of the human substantial form, also known as the soul. Aristotle taught that the soul is “the substantial form” of a living being; Condic quotes his famous definition of substantial form: “the principle of act in relation to prime matter that makes something be what it is, most fundamentally.” She then wryly remarks that modern people will “struggle with such an abstract definition.” Fair enough, and the account she develops of substantial form is quite interesting, as we will see. But let us first note some striking features in Aristotle’s definition. A soul is not a thing per se, but a “principle.” It has to do with actions, and particularly those actions that speak “most fundamentally” of what the living being is. How might a modern understand this “principle of act”?16
Suppose you have, on the one hand, a living human being, and on the other, “a simple pile of organic molecules” that are identical to those in the human. What is the difference that makes the former alive and human? It cannot be the molecules—those are, by hypothesis, the same. Rather, the difference is found in how the material is “formed” or “organized.” “Substantial form is the cause of the observed organization” of a human being, which is to say that “the matter comprising a human is ordered by a specific set of rules that is distinct from the rules ordering those same molecules in a non-living pile.” The important point is that no stuff has been added. Condic notes, as an aside, that Mary Shelley has bequeathed us a misleading picture, for she has Frankenstein add mysterious stuff—electricity—to his creature to bring it to life. The living difference, the difference of being alive, is in having a substantial form, a specific set of rules, something which we can identify by observation of the entity even though it is not any sort of stuff that has been added to the entity.17
In concrete, biological terms, she explains, “these rules [are] practical matters of ligan-receptor binding affinities, concentration gradients, intracellular diffusion constants and other principles that govern how molecules function within cells and how cells communicate during formation of the mature body.” Taken together, these “rules or principles . . . constitute the substantial form of the individual.” There is nothing mystical about substantial form. In principle, she notes, if it were possible to organize the various chemicals in the right way— chemicals that start out lifeless, inert—one would be creating “a soul that would subsequently be the cause of all the ongoing properties of life observed in the entity thus constituted.”18
The question of the human soul (as opposed to a merely animal soul) is, as Condic notes, a special case in both Aristotle and Aquinas about which there is more to be said. But here she has given us at least, she says, the idea of an animal substantial form in familiar, modern terms. I take her reticence on special human creation to be instructive. We need not conflate the question of human distinctiveness with the question of whether a being has a human soul. Within theology, it is a profoundly important matter whether each human being is the result of a particular and new divine creative act—which is roughly the view of Aquinas, and which, he says, is not true of other living beings when they come to be. But whether specially created or no, Condic wants us to see on scientific terms that the human substantial form is there in the zygote and thus we have a human zygote, a human being.
These rules and principles that collectively constitute the human substantial form are what enforms the matter (the chemicals and so forth) of a human being throughout his or her life. When, then, does a human embryo give evidence of having a substantial form? As we have noted above, it happens in the first seconds following the fusion of sperm and oocyte: the organization of the zygote which begins the human developmental process.
A tumor or a “mole” (she uses as an example “a complete hydatidiform mole” or CHM, which is formed by the fertilization of an egg that lacks its nucleus) also has a substantial form, but this form is not human but rather cellular. A CHM never acts as a whole organism, never shows integration as a whole; by contrast with an embryo, it remains merely a group of cells.
Thus we see the reason for affirming there is a human soul present from the initial one-celled human zygote. To affirm that is not to invoke non-scientific mystery but merely to note the rules and principles embodied already in the organization of that living entity—to take note of its substantial form.
V. Twinning and Talk About Souls
The language of souls is deep in our consciousness in the West, yet Condic’s argument would not be the first to suggest that we might be able to do without it. The Christian philosopher Nancey Murphy puts the conclusion provocatively when she tells Christians they do not need to think that they have a thing called a soul.19 Murphy’s argument includes both biblical interpretation and scientific data. Broadly speaking, the Bible describes human beings as “spirited bodies,” she says, rather than fusions of two things, bodies and souls. From science, Murphy draws on the evidence of causality working not only from the bottom up but also from the top down—not only, that is, from constituent parts to wholes but also from wholes down to parts. Importantly, top-down causality is not mechanical or (reductively) material. Murphy’s arguments map quite easily onto Condic’s modern account of substantial form.
Murphy’s view is called “non-reductive monism.” It is a monism because it does not hold that there are ultimately two parts to a human being (body and soul) but rather that we are one substantial thing. But importantly it is also non-reductive: The activity of a human being is not reduced to the purposeless interaction of his or her constitutive parts. Do your neurons make you do what you do? No, but not because there is something else there, but rather because our complexity does not reduce to our individual cells and their chemicals. I could not be typing these words if my neurons were not in working order. Nor could I be typing these words if I lacked a human substantial form, the rules and principles that organize my body’s parts.
Why should we even think about the soul when we are considering twinning? It is precisely because many people have spoken of human beings as being two things that are put together: a body and a soul. And if the human soul is something extra, some “thing” that is added to the human body, then one might find abortion acceptable, particularly if there is a period between the physical beginning of human life and the “ensoulment” of a human being.20 A proper understanding of the soul is exceedingly difficult: To go back, Aristotle clearly does not speak of it as a ghost or a spirit or anything we might try to imagine, but rather as a certain kind of active principle that has to do with what the entity in question most deeply is. But whatever the soul is, we should never think of it as being added to a human being—just as, although it is a different argument, we should not think there are human beings who are not human persons.
It emphasizes human dignity to speak of human beings as “having” souls, but in light of the complexity of that subject I am impressed also with the way older language often spoke of human beings just as “souls,” full stop. Philosophically and scientifically difficult as these investigations are, they also, properly, bring us to a place of awe. The mystery of our being is that we are complex unities. We are “fearfully and wonderfully made,” as the Psalmist says, and continues: “marvellous are thy works, and that my soul knoweth right well.” And so we have been ever since sperm united with oocyte.21
1. Classical Christian moral teaching is that, for a killing to be justifiable, the party must be guilty of an offense of serious gravity, and even then the killing may be done only by an agent of the state. Whether these strictures need modification when we consider abortion has been considered at length by moral theologians. But given that our context is one of widespread acceptance of abortion, and given also that the acceptance of abortion has been with us for a couple of generations, I think it better to sideline this question for now. It is enough already, in fact quite a lot, to recognize that we are speaking of human beings, as I will go on to say, all the way back to the initial single-cell human organism.
2. Gilbert Meilaender, Bioethics: A Primer for Christians (Grand Rapids: Eerdmans, 1996), 30–31.
3. It may be that a number of these entities are not embryos strictly speaking, that is to say, they lack the organizational capacity to develop as living beings (they might be tumors, for instance). Or alternatively, and in addition, it could be that our own conception of the length and fragility of a human life is what needs adjusting. I am grateful to Dr. Meilaender for personal correspondence on this matter.
4. Gilbert Meilaender, Bioethics: A Primer for Christians, 3rd ed. (Grand Rapids: Eerdmans, 2013), 31. The quotations in this paragraph are from the third edition, but the second or fourth could equally be cited.
5. Maureen L. Condic, Untangling Twinning: What Science Tells Us About the Nature of Human Embryos (Notre Dame: University of Notre Dame Press, 2020).
6. Quotations in this paragraph from Condic, 8-9.
7. Quotations in this paragraph from Condic, 9-11.
8. Condic, 44.
9. This may not be precisely true for embryos beyond the two-cell stage. Condic emphasizes that “totipotency is a property for a single cell,” and totipotency means “capable of developing into a complete organism”—a stronger claim than to say capable of differentiating into any cell or tissue of a complete organism. (Condic, 13). For the benefit of the argument, however, we take the harder case.
10. Quotations in this paragraph from Condic, 45.
11. The language of “external hazard” is mine, not Condic’s.
12. Quotations in this paragraph from Condic, 47–48.
13. Quotations in this paragraph from Condic, 54.
14. Condic, 55.
15. Quotations in this paragraph from Condic, 56–57.
16. Quotations in this paragraph from Condic, 88.
17. Quotations in this paragraph from Condic, 88–89. Condic’s dissent from Shelley’s imagination that the creature comes alive when the stuff of electricity is added is a philosophical point: To be alive is not to have had something added. There is no dissent from the novel’s moral warning against the presumption to manufacture a human being.
18. Quotations in this paragraph from Condic, 89.
19. See Nancey Murphy, Bodies and Souls, or Spirited Bodies? (Cambridge: Cambridge University Press, 2006).
20. An instance of this is in philosopher Robert Pasnau, Thomas Aquinas on Human Nature (Cambridge: Cambridge University Press, 2002), 108: “[For Aquinas,] actual human life does not begin until well after conception. The developing fetus does not count as a human being until it possesses a human soul, and this does not occur until the fetus has developed its brain and sensory systems to the point where it can support the distinctive intellectual capacities of a human being. We can attack the pro-life position at its weakest point: at its claim that an unformed mass of cells can genuinely count as a human being.” Condic shows us that, to the contrary, the fetus (and the embryo!) is never “an unformed mass of cells.”
21. Psalm 139:13 (Book of Common Prayer ).
Rev. Canon Victor Lee Austin is Theologian-in-Residence of the Episcopal Diocese of Dallas, and author most recently of Friendship: The Heart of Being Human.