The Moral Status of the Human Embryo: The Twinning Argument. (the New Bioethics)
Brogaard, Berit, Free Inquiry
Recent scientific advances in research involving stem cells derived from human embryos have sparked considerable ethical debate concerning the moral status of the human embryo. Scientists believe that research using stem cells might eventually help us cure diseases such as Alzheimer's, Parkinson's, juvenile diabetes, spinal cord injury, and diseases of main organs. Stem cells apparently have the capacity to transform themselves into specific organ tissues. In the future, researchers may be able to use stem cells to develop, for example, liver cells that could cure someone with a malfunctioning liver. Because human embryonic stem cells have the potential to develop into all of the tissues in the human body, stem cells from human embryos are believed to have a greater potential to produce these results than stem cells from adult cells, umbilical cords, or the human placenta.
For research, embryonic stem cells are isolated from human blastocysts--embryos at around day four in the fetal development. In the course of obtaining stem cells from a living human blastocyst, the blastocyst is destroyed. For that reason, the main ethical challenge associated with stem cell research has to do with whether blastocysts have any moral status. There are two main arguments for the view that blastocysts have moral status. One relies on the claim that, if the embryo under favorable circumstances would be identical to the human being as it would exist after birth, then it already has the same moral status as a human being after birth. The other relies on the claim that the embryo is entitled to protection because from the moment of conception it is a potential human being, even if it would not be identical to the future human being under favorable circumstances. The second of these arguments will not concern me here. (1) My focus is on the argument that the embryo has moral status because under fav orable circumstances it would be identical to the human being that would exist after birth. I shall argue that the premise is false.
The most important argument against this claim, I believe, is the twinning argument. (2) The twinning argument, in its simplest form, is familiar: prior to gastrulation (the critical developmental step at which the embryo's three major germ layers, or tissue type divisions, form), the embryo is susceptible to twinning. Accordingly, it has the potential to develop into several human beings. Since the process of becoming a human being has not yet ended, the pre-gastrular embryo is not a human being.3 Unfortunately, the twinning argument in its simplest form is flawed. The premise that if an entity is potentially two, it cannot be one is false. As the American Civil War teaches us, there are cases where identity is inherited even though an entity is susceptible to twinning. The United States in the period immediately prior to the Civil War was actually one but potentially two. (4) But, I will now argue, even though there are clear cases of individuals susceptible to twinning that persist over time, there are emp irical grounds for denying that an embryo susceptible to twinning can be transtemporally identical to a future human being.
To make a case for the proposal that an embryo susceptible to twinning cannot be identical to a future human being, I shall first consider the possible scenarios under which twinning might occur. Second, for each possible scenario I shall determine whether there are empirical grounds for thinking that human embryonic twinning occurs under that scenario. Finally for each possible scenario I shall determine whether it is conceivable that the pre-gastrular embryo under favorable circumstances is identical to the human being as it exists after birth. As we will see, according to the scenario under which embryonic twinning is known to occur, it is impossible that the pre-gastrular embryo is identical to a future human being.
How SHALL WE UNDERSTAND TWINNING?
First, one may hold that twinning is a form of cloning. On this view, the embryo would, in some pre-gastrular phase, be such that a process of forming new human individuals can still occur via budding. When budding occurs, a part of one individual substance becomes detached and forms a new individual substance in its own right, while the original substance continues to exist. Similar scenarios are known from the vegetable kingdom, where a cutting from one plant may be planted in the soil to result in a new plant without the original plant ceasing to exist as a separate individual. Budding does not entail the destruction of the entity that buds off the new individual. So, if embryonic twinning occurs via budding, the view that the pre-gastrular embryo under favorable circumstances would be transtemporally identical to the future human being is credible. Unfortunately however, human development is nothing like that of plants. When a cutting is taken from a fully developed plant we have an original organism and a part that develops into a separate individual. We do not have one cell, or mass of cells, which divides into two. But this is exactly what we have in the case of embryos that undergo twinning.
Second, in light of the above considerations, one may hold that twinning occurs via fission. (5) When an entity (for example, an amoeba) undergoes fission, the entity reduplicates itself. New parts are formed that then split apart to lead separate existences. When fission occurs, the entity that undergoes fission is destroyed. This claim rests on the conceptual thesis that one individual cannot be identical to two individuals. Think of a log that is split into two for burning in the fireplace. One may find it quite natural to think that one log is identical with two logs given that their time frames are different. However, identity over time, like all other sorts of identity, is transitive. That is to say if John-as-a-baby is identical to John-as-a-child, and John-as-a-child is identical to John-as-an-adult, then John-as-a-baby is identical to John-as-an-adult. Identity over time is also symmetric: if John-as-a-child is identical to John-as-an-adult, then John-as-an-adult is identical to John-as-a-child. Comp are identity to love which is neither transitive nor symmetric. From the fact that John loves Mary and Mary loves Peter, it does not fob low that John loves Peter. And from the fact that John loves Mary, it does not follow that Mary loves John. Since identity is both transitive and symmetric, if the one log were identical to the two logs, the two logs would be identical to each other. But they are not. We just said they were two, not one.
Of course, one may say that the one log is identical to the two logs taken together. But if this is so, then spatial connectedness does not matter in defining what a log is. That is, something can be a log even if its parts are spatially separated. In the case of an embryo spatial separation may not matter either. But the two embryos that would result from the splitting of a single embryo would under normal circumstances develop into two human beings. So if the single embryo were identical to the two embryos taken together, then the single embryo would be identical to two adult human beings, which cannot be the case. Alternatively one but not the other of the twins would be identical to the ancestor entity. But there is no property in virtue of which one but not the other twin could be said to be identical to the ancestor entity. So, if human embryonic twinning occurs via fission, an embryo that results from a twinning process begins to exist when the twinning process is completed.
But the fact that an embryo ceases to exist when twinning occurs does not rule out the possibility that an embryo that does not undergo twinning is identical to a future human being. So, one may hold the view that, when twinning occurs, the embryo begins to exist when the twinning process is completed, but, when twinning does not occur, the embryo begins to exist at the time of conception. Since we do not know whether a blastocyst would undergo twinning under favorable circumstances, we do not know whether it would be identical to a future human being under favorable circumstances. And since we do not know whether the blastocyst has moral status, destruction of the blastocyst is not justified.
The aforementioned theory entails that I could not have been a twin if I am not in fact a twin. For if twinning had occurred to me, I would have ceased to exist, and two new human individuals, that are not identical to me, would have come into existence. It may seem counter intuitive that I could not have been a twin. But no objection to the possibility that a pre-gastrular embryo--as twinning falls to occur--is identical to a future human being is to be found here. It is simply a conceptual fact that individuals susceptible to twinning via fission could not have been twins.
As it turns out, however, human embryos do not undergo twinning via fission. The reason is that the cells--and sums of cells--in the pre-gastrular embryo are totipotent: each of them has the potential to develop into a complete human being. The parts of an amoeba or a virus, by contrast, do not have the potential to develop into a new organism. New parts are formed which then spilt apart to lead separate existences. When the pre-gastrular embryo undergoes twinning, the bits destined to become a human being split into two halves without any foregoing reduplication. (6)
Recent embryonic research has given rise to skepticism about the view that the embryo is a featureless orb of cells prior to gastrulation. It has been shown that the mammalian body plan begins to be laid down already from the moment of conception. (7) A body axis is present, and there is already a clear division between bits destined to become the placenta (etc.) and bits destined to become the future human being, even at the two-cell zygote stage. Despite the existence of traits that appear to narrow down cell fate, the fact is that pre-gastrular embryos are susceptible to twinning. Even though the cells become biased towards producing certain tissues, those biases are not irrevocable. So the new data do not dispute the claim that the cells destined to become a human being are totipotent. After all, the pre-gastrular embryo does not have the kind of structure that would prevent it from separating into parts that can each produce a complete human being.
SEPARATION AS THE WAY TWINNING OCCURS
This leads us to the third way in which twinning can occur. On this scenario, the embryo is already not one but two (or more) entities, both of which would survive, should twinning occur, to form two independent embryos. That is, the embryo is, in some pre-gastrular phase, such that a process of forming new human individuals can still occur via separation. Separation is distinct from fission in that two or more entities are joined together as one entity and at some point the relations conjoining the parts of this entity are disrupted in such a way that the previously attached individuals continue as separate new substances. In the case where twinning does not occur, one may hold one of the following views: one of the two parts of the embryo is under favorable circumstances transtemporally identical to the human being that exists after birth. Alternatively, one may hold that both parts of the embryo in favorable circumstances are identical to the human being.
The first of these alternatives implies a peculiar process under which--as twinning falls to occur--one human being would absorb into itself another entity that is of exactly analogous form and structure. But more important, it leaves open the question as to what might make it true that one but not the other half of the total embryo as it exists prior to gastrulation should be the human being that exists after birth.
On the second alternative, an embryo that does not undergo twinning consists of several parts that in favorable circumstances are jointly identical to the future human being. To show that this alternative is not possible either, I need to employ the notion of being identical to a human being whose possibility was never realized. The notion of being identical to all sorts of possible human beings is not strange at all. If I had not left my hometown I would not have gone to Buffalo nor have married Joe. If I had gone to Ohio State University instead of the University at Buffalo, I would have met Joe earlier than I actually did. These conditionals are not true of anyone else but me, and they are true of me. So, I am identical to a possible person that was never actualized, given my decision to leave my hometown and attend the University at Buffalo. Notice, however, that for each possible scenario there is only one possible human being to which I am identical, never two or three. So I am identical to just one pos sible human being in the possible situation in which I go to Ohio State University instead of the University at Buffalo. That is, I am identical to the possible human being (in that possible situation) with whom I share most of my qualities in common, including the quality of having originated from particular sperm and egg cells.
I can now turn to my argument that the second alternative is not possible either. Call the two halves of the embryo prior to gastrulation when twinning is still possible A and B. Even though twinning does not in fact occur, there is a possible situation in which A and B separate and continue as separate human beings. The actual human being that exists after birth is transtemporally identical to the actual sum of A and B. But the actual sum of A and B is identical to the possible sum of A and B. The possible sum of A and B, on the other hand, is identical to the possible sum of the two possible human beings. By transitivity, it follows that an actual human being is identical to two possible human beings that are both located in one and the same possible situation. But one human being cannot be identical to two possible human beings that are both located in one and the same possible situation. So the second alternative is not possible either.
An opponent may now object that the bundle of cells constituting the embryo is a whole, and that spatial separation of the two halves of a bundle is enough to destroy the whole. So, even for the case of twinning by separation, the loosely connected bundle of cells is a whole that ceases to exist if twinning occurs. But then the possible sum of A and B is not identical to the two possible human beings. So, the aforementioned transitivity of identity is averted.
What counts against this proposal is that the parts of the pre-gastrular embryo destined to become the human being and the parts destined to become the placenta are not spatially separated. If we suppose the degree of spatial separation between two entities x and y determines whether x is identical to a future individual z while y is identical to a future individual v, the parts of the embryo destined to become the human being would in every case fail to be identical to the human being.
Perhaps the objection is rather this: that there are cases where separation happens because the internal relations among certain parts of a whole cease to obtain, resulting in a split along the line where the former internal relations no longer hold. Even if we think of the pre-split thing as consisting of two parts, it is still one whole thing (and not a mere collection of parts). Because of the nature of identity, the original entity cannot be identical to both of the resultant entities.
The problem with this proposal is that the cells in the early embryo form a mere mass, being kept together spatially by an outer membrane. There is no causal interaction between the cells. They are separate bodies, which adhere to each other through their sticky surfaces and which have at this point only the bare capacity for dividing. In view of that, there is no reason to think that the entire bundle of cells, but not any one part of it, has some property in virtue of which it is identical to some potential human being. If each of the parts with a potential to become a complete human being is identical to a potential human being, the inference goes through.
In sum, human embryonic twinning occurs via separation. But if twinning occurs via separation, an embryo susceptible to twinning cannot be transtemporally identical to a future human being. Hence, if blastocysts are worth protecting, it is not because of their prospective identity to entities that we already know are worth protecting. (8)
(1.) Singer and Dawson have discusscd this argument in P. Singer, K. Dawson, "IVF Technology and the Argument from Potential," Philosophy and Public Affairs 17 (1988:87-104). See also J. Feinberg, "Abortion," in Tom Regan, ed., Matters of Life and Death: New Introductory Essays in Moral Philosophy, 2nd ed. (New York: Random House 1980, 1986), p. 267; and D. Hershenov, "The Problem of Potentiality" Public Affairs Quarterly 13 (1999): 255-71.
(2.) In "Sixteen Days" (The Journal of Medicine and Philosophy, forthcoming) Barry Smith and I argue that the pre-gastrular human embryo is not a human being. In this note I develop a new version of the twinning argument that is in many ways stronger than the twinning argument presented there. For discussion of the simple version of the twinning argument, see N.M. Ford, When Did I Begin? Conception of the Human Individual in History, Philosophy, and Science (Cambridge/New York: Cambridge University Press, 1988).
(3.) L.C. Becker, "Human Being: The Boundaries of the Concept," Philosophy and Public Affairs 4, (1975): 340.
(4.) Most likely, when we say that the United States is potentially two, we mean that a part of the United States could have become detached and formed a new country in its own right while the United States would have continued to exist.
(5.) W. Quinn, "Abortion: Identity and Loss," Philosophy and Public Affairs 13 (1984): 27.
(6.) Of course, as any horticulturalist knows, many plants can be bisected with the bisected parts developing into complete plants of the same species. Similarly Planaria are noted for their great ability to regenerate missing body parts. Chop a planarian into two pieces and each piece will develop the missing parts and become a separate planarian; the bead of one (or the left side) is identical to the corresponding part of the original; the tail of the other (or the right side) is identical to the tall of the original. If the bisection of plants and flatworms is a form of twinning by fission, then we have to go carefully to distinguish what we are saying about human twinning. For if plants and flatworms, both of which have parts that are totipotent, can undergo twinning by fission, then what reason do we have for thinking that human embryos, which have parts that are totipotent, do not undergo twinning by fission? Granted. However, these cases appear to be cases of twinning by separation (see below). After a ll, if I can cut a flatworm into two pieces that can both survive, then it would seem that each part of the flatworm that can survive on its own is identical to a potential flatworm. But this means that flatworms and plants the parts of which can continue to exist as separate entities are not individuals in the sense in which human beings are individuals. Rather, they are collections of parts that could survive on their own under the right circumstances.
(7.) "Your Destiny From Day One," Nature Science Update, 8 July 2002.
(8.) Thanks to Judy Crane, John Danley, David Hershenov, Richard Hull, Joe Salerno, Ron Sandler, and Rob Ware for helpful comments.
Berit Brogaard is assistant professor of philosophy at the Southern Illinois University-Edwardsville.…
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Publication information: Article title: The Moral Status of the Human Embryo: The Twinning Argument. (the New Bioethics). Contributors: Brogaard, Berit - Author. Magazine title: Free Inquiry. Volume: 23. Issue: 1 Publication date: Winter 2002. Page number: 45+. © 1999 Council for Democratic and Secular Humanism, Inc. COPYRIGHT 2002 Gale Group.
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