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Marquis, Pruss and the Twinning Argument

March 23rd, 2009 by Matt

Augustine writes,

And therefore the following question may be very carefully inquired into and discussed by learned men, though I do not know whether it is in man’s power to resolve it: At what time the infant begins to live in the womb: whether life exists in a latent form before it manifests itself in the motions of the living being.
Augustine of Hippo, The Enchiridion, 85.

I think Augustine’s comments ring true today. Like Augustine I believe there is good reason for thinking a fetus (a formed conceptus) is a human being and like Augustine I oppose destroying the conceptus at any stage in pregnancy. I am also inclined to agree with Augustine that prior to formation, the best stance to adopt is agnosticism if we do not know when the human being begins to exist; I have argued for this elsewhere.

Madeleine asked Alexander Pruss to either help her understand or overcome an argument advanced by Don Marquis that had given rise to my agnosticism regarding the point at which a human being comes into existence (human being is defined as an organism, the killing of which constitutes homicide).

My agnosticism is based on three things. The first is that I know of no reason for thinking an embryo is a human being at the time of conception. The second is that two arguments against this conclusion lead me to wonder if we can know this. The first is the twinning argument. The second is Don Marquis recent track-forward argument.

Pruss, in his post, only addresses the twinning argument. Having read Pruss’s argument I think that he and I are in fundamental agreement; moreover, I think he provides reasons for accepting an agnostic position, at least with regard to identical twins.

First, a brief summary of the twinning argument is necessary to give my readers some clarity as to what we are talking about.

Prior to the time when an embryo receives a primitive streak, the group of cells which indicate the basic body plan of the embryo, an embryo can twin. Occasionally monozygotic twinning occurs and a single embryo splits to form two or more separate embryos. Many Theologians and Philosophers have argued that this fact provides strong evidence that human beings do not come into existence prior to 14 days post conception.

The argument goes as follows. Suppose a human being comes into existence at conception, let’s call this embryo Bob. Bob symmetrically splits into two forming two embryos, embryo A and embryo B. The question arises, what happened to Bob?

There are only four possibilities.

(a) Bob ceased to exist and two new individual human beings come into existence at twinning;
(b) Bob continued as embryo A and embryo B came into existence at twinning;
(c) Bob continues as embryo B and embryo A came into existence at twinning;
(d) Bob is identical with both embryo A and embryo B.

The objector argues that each of these options is acceptable.

Pruss gives two main responses to this argument. The first is to deny that all of the options (a) (b) (c) and (d) are all unacceptable. The second is to call into question whether symmetric twinning actually occurs.

Are (a) (b) (c) and (d) all Unacceptable Options?
Pruss does not argue for the acceptability of (d) and for good reason, (d) leads to a straightforward contradiction due to the transitivity of identity. According to the transitivity of identity, if an object x is identical to another object y and y is identical to x then x is identical z.

In (d) embryo A is identical with Bob, Bob is also identical with embryo B, hence by transitivity of identity, Embryo A should be the same organism as embryo B but clearly is not; as I noted, two separate embryos came into existence at twinning and two separate organisms cannot be the same organism.

As I stated, Pruss does not address this but he does defends the possibility of (a) (b) and (c).

Let’s turn first to (a); Pruss here suggests that this option is “not absurd” and gives an example of an amoeba; Pruss notes that when an amoeba splits into two we normally think the original one is destroyed and two new ones come into existence. So when the conceptus twins, the original one is destroyed and two new organisms come into existence.

Jeff McMahan and Peter Singer have offered a response to this line of argument. McMahan suggests adopting (a) “is an embarrassment” for anyone who opposes abortion on the grounds that life begins at conception because they are committed to the claim that “it’s a terrible loss when one of us dies in utero;” hence, anyone who thinks hominisation (the coming into existence of a human being) occurs at conception is committed to viewing monozygotic twinning as bad and as tragic as the death of a newborn infant due to the demise of the original conceptus.

Now I am not convinced by this line of reasoning entirely; if a newborn split into two and two new, genetically identical, infants were formed, it is not clear to me that we would consider this occurrence tragic, our attitudes to infants notwithstanding.

Similarly, with Pruss’ brain splitting example, I don’t think that if we conducted a brain splitting operation on a human being and placed the two hemispheres of the brain into two genetically identical bodies we would not consider this to be on par with homicide, despite the fact that we admit that an adult human is human.

Now, if cases like this happened we would be genuinely perplexed as these cases are extremely strange and bizarre, if they could happen we would end up with two beings that do not differ, either psychologically or physiologically, from the being of origin. So I agree with Pruss that this situation is possible and is not clearly or obviously unacceptable.

However, I find it interesting that while Pruss defends this option as acceptable he backs away from endorsing it; he states that “if x in fact is going to symmetrically split in the near future, then maybe x is not identical with any far-future entity”[Emphasis added] he then refers the reader to his discussion of the other two cases. I think he is correct to do this; to show that an option is not false is not to show there are any reasons for thinking it is true.

Turning now to (b) and (c); the standard argument against adopting either claim is that to do so is arbitrary, one cannot provide any reason for identifying Bob with embryo A that is not also a reason for identifying Bob with embryo B. Pruss response is to argue

There might be some law specifying which of y and z gets x‘s soul, either in terms of some minor asymmetry (nobody thinks the asymmetry is total, with each half having the exact same number of molecules, in exactly the same positions) or stochastically (maybe it’s random where the soul goes), with the other output entity getting a new soul. Or it might be that God decides where x‘s soul goes.

Now on the face of it this does not appear to address the objection; the objector argues that one can never be rational in affirming that (b) is the case without also affirming (c). Pruss provides no reason for saying you can. His position is simply that either option remains possible.

I think something important can be ascertained from Pruss’ objection here and that is that the objector does not show; that either (b) or (c) are false. At best one can only show that that we cannot affirm either option nor can we deny either option has occurred. What the objector tells us here is that we cannot know at this stage.

It we look at Pruss’ response then it seems what he tells us is that when we ask the question regarding a set of monozygotic twins, did they come into existence at conception? we do not know. It is possible they did not both come into existence when twinning occurred and it is possible that one of the two began at conception though we do not know whether this is the case and even if we did, we don’t know which one of the two was prior.

Does Symmetric Twinning Occur?
Similar conclusions apply to the other main argument Pruss provides against the twinning argument. He writes,

last time I checked, we did not actually know that embryonic splitting is in fact symmetric. If it turns out that embryonic splitting proceeds by budding, the argument falls flat. Thus, the argument rests on an empirical hypothesis which is merely speculative. This is a problem: obviously, if the case for the lack of a right to life on the part of some organism is based on a merely speculative hypothesis, we should treat the organism as if it had a right to life until that speculative hypothesis is checked.

Here Pruss argues that we do not know whether symmetrical twinning actually occurs and he argues that in the absence of any reason for denying that an embryo is human we should treat it as though it is. Now I don’t deny the first premise, all it does is again point out that when we are dealing with monozgotic twins, we do not know whether either or both came into existence at conception, as we do not know whether twinning is symmetric, where the above puzzles arise, or asymmetric, whereas Pruss argues they do not.

Pruss goes on to argue that when we don’t know we should treat the embryo as though it has a right to life. While I do not endorse this claim as it is phrased, I argued for a similar claim in my post; I argued that in cases where we do not know if an embryo is a human being it would be reckless and hence, gravely wrong to kill it because it might be.

So in conclusion, I think that the twinning argument should give us pause about claiming that human beings come into existence at conception. We do not know whether twinning is symmetric or asymmetric. If it is the latter, we do not know whether anyone who is a monozygotic twin came into existence or conception or not. Either option is a possibility but we do not have the information to know which is option is correct.

RELATED POSTS:
Some Thoughts on Human Embryonic Stem-cell Research
Is Abortion Liberal? Part 1
Is Abortion Liberal? Part 2
Sentience Part 1
Sentience Part 2
Viability
Abortion and Brain Death: A Response to Farrar
Abortion and Capital Punishment: No Contradiction
Imposing You Beliefs Onto Others: A Defence
Published: Boonin’s Defense of the Sentience Criteria – A Critique
Published: Abortion and Capital Punishment – No Contradiction

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23 responses so far ↓

  • Matt, you base your entire analysis on the idea that an entity can at one time be human but not a person.

    Take a step back. Where does this idea come from? Does it really make any sense?

    Scientifically (genetically) human life starts at conception. You wish to propose that human “personhood”, or whatever you want to call it, whatever that is, starts later.

    I don’t know how you approach questions like this philosophically, I’m used to thinking scientifically, so here is my reasoning:

    Start with a null hypothesis. This is the simplest option – ie that human life and personhood are the same thing and start at conception.

    The alternative hypothesis is that human life starts at conception and “personhood” starts later.

    Now I must look for evidence for the alternative hypothesis. And frankly, I can find none. There is no biblical explanation of when the soul is imparted. And anyway, when the soul is imparted is probably irrelevant – Christ himself points out in Matthew 10:28 that humans can only kill the body, not the soul. The command “Thou shalt not murder” can only therefore refer to killing the human body, not the soul. So the soul is irrelevant to the current debate, as we know the embryo is (genetically) a human body that can be killed.

    Nor is there any scientific reason to believe “personhood” starts later than conception. There is no other point during development when the embryo makes a major change such as that at conception, development after that is gradual and linear.

    So, in the lack of any evidence to the contrary, I must conclude the null hypothesis is correct – ie human life and personhood are the same thing, and start at conception. And ending this human life is a breach of the 6th commandment, right back to conception.

    The twinning argument does not present any evidence for the alternative hypothesis, just adds a little bit of interesting confusion to the issue for ONLY those few babies that happen to twin – which is rare, how many identical twins do you know? It is a minor side-issue that is interesting to discuss but ultimately irrelevant, especially as it is inconclusive in either direction.

    I have outlined my views on personhood and the twinning argument in the following two posts:
    Is an embryo a person?
    Is an embryo a person 2 – Twinning

  • I have no disagreement with the claim that when x is a monozygotic twin, we might not know whether x came into existence at fertilization or not.

    However, I do not see why that should translate to an agnosticism about the beginning of existence of people who are not monozygotic twins.

    If x is an amoeba, then perhaps I need to be agnostic about whether x came into existence at the last splitting (this seems most likely) or earlier (so that the last splitting was metaphysically a budding off of an offspring).

    But if [t0,t1] is any interval of times in a continuous amoeba life during which time no budding/splitting/death occurred, then I can be pretty sure that I have the same amoeba at t1 as at t0. This is a sufficient, not a necessary, condition for identity.

    In the case of human beings, when x is not in fact a monozygotic twin, and x is still alive, [fertilization,now] is an interval of times in a continuous human life during which time no budding/splitting/death occurred. So, by analogy, we have the same human being during that time. If x a monozygotic twin, and t0 is any time after splitting, then only [t0,now] is such an interval, and so the criterion only tells us that after splitting we’ve got the same human being as we eventually do.

    I do think that if twinning is death for the original embryo, then it is a tragedy. We don’t feel about it as such, maybe? Well, (a) it’s a tragedy associated with a joy (one person dies, asexually reproducing into two persons), and (b) as many events in human history illustrate, we have a hard time sympathising with people who don’t look like us.

    There comes an age (different for different people) at which people cease to be capable of sexual reproduction. We are not tempted to say that at this age we have a new entity. The primitive streak is an age at which people cease to be capable of asexual reproduction. This may seem more significant than a cessation of the capability of sexual reproduction, but note this: if an amoeba lost its ability to asexually reproduce, we would not have the least temptation to consider the moment of that loss of ability as the first moment of a new being’s existence.

    I think we shouldn’t multiply kinds of beings beyond necessity. There is no need, thus, to posit a special kind of organism that exists only for a few days after fertilization. (Positing an individual of an already known kind–homo sapiens–that exists only for a few days after fertilization is not a problem.)

  • Hi Alexander Pruss,

    I am honoured to have you comment on my blog.

    You write,

    I have no disagreement with the claim that when x is a monozygotic twin, we might not know whether x came into existence at fertilization or not. However, I do not see why that should translate to an agnosticism about the beginning of existence of people who are not monozygotic twins

    I agree, perhaps my blog was not clear, but I think the twinning argument only gives us reason to be agnostic about whether twins came into existence at conception. An agnosticism beyond this would be based on factors other than the twinning argument, such as the splitting of a single celled zygote post conception or an inability to find a cogent argument either for or against the thesis that humans come into existence at conception. Issues which go beyond the twinning argument.

    You write

    I do think that if twinning is death for the original embryo, then it is a tragedy. We don’t feel about it as such, maybe? Well, (a) it’s a tragedy associated with a joy (one person dies, asexually reproducing into two persons), and (b) as many events in human history illustrate, we have a hard time sympathising with people who don’t look like us.

    I also agree with you regarding the issue of how we react. It seems to be we do not grieve every time a human being dies, humans die every day and most of the time we do not react adversely, its only when a person close to us, whom we know, dies that we tend to feel grief. Moreover, as I said in my argument I think a closer analogy would be newborn infants engaging in symmetrical twinning. If this occurred regularly I doubt we would grieve the loss of the original infant but it would not commit us to denying the humanity of the infant.

  • Pruss gives two main responses to this argument. The first is to deny that all of the options (a) (b) (c) and (d) are all unacceptable.

    While this sentence makes sense, it could be unconvoluted to help understanding 🙂

    I think these are reasonable questions. The case of twinning, while an exception, is still useful to help us understand the implantation of the soul; that is its rarity cannot be used to dismiss the argument.

    The twins may not be identical however. It possibly not the classical genes that are decisive, rather meta-information supra-coded in the genome that tell the embryo how to develop. This epigenetic information varies according to both classic coding and methylation of the bases. The methylation and demethylation varies according to specific tissue, how do we know that the cells in the embryonic stage are identical? If not identical then one can at least argue that the split embryos are somewhat different.

    This doesn’t resolve the argument but gives added information. The fact that we can grow and split embryos ad-infinitum in the laboratory does cause one pause.

    While the science adds to our understanding, personally I think this argument is going to be resolved theologically.

  • bethyada: “how do we know that the cells in the embryonic stage are identical”
    The genetic information is the same, what information is being used is not. It is unlikely that the two would be exactly identical – they may have different numbers of cells for example, and those cells could be slightly different. But the genetic information is the same, which is why you get identical twins.

    This is how identical twins are formed. Non-identical twins occur when two eggs are released and both fertilised, developing separately.

    I don’t know that twinning does help us “understand the implantation of the soul” – we have a number of options still and no way of knowing which one is true!

  • The genetic information is the same, what information is being used is not. It is unlikely that the two would be exactly identical – they may have different numbers of cells for example, and those cells could be slightly different. But the genetic information is the same, which is why you get identical twins.

    This is how identical twins are formed. Non-identical twins occur when two eggs are released and both fertilised, developing separately.

    Yes, I am aware of the difference between identical and fraternal twins.

    I am saying that although identical twins start with the same genetic information, some genetic changes may take place. Further, the alteration of the genome in the form of methylation of the bases (adding CH3 to cysteine) alters the way genes are expressed. It is likely that much of the information in our genomes in terms of this epigenetics is much more significant in both our humanness, and quite possibly our personalities.

  • My comment would probably seem out of place in such a well considered blog.

    A simple man like myself might say that it appears that human beings came into existence at conception or in the case of asymmetric twinning at the point of twinning.

    My mother was with child and she expected and delivered a human child (lucky for her and me). Do we need to consider if a human embryo is a human being, if not a human being what other type of being does it purports to be?

    The crux of the matter is this. A human embryo is likely to be born if you leave it alone. It lives and is human, however if you would like to kill it than you need to introduce your argument in terms of “does it have a right to live”.

    The problem created by your intent to kill another human is that you yourself would like the right to live. So how do you protect yourself and at the same time allow yourself to kill another. Since you granted the “right to live” to human beings (this grant is not scientific by the way) you now need to turn your victim into a non-human.

    Now science is burdened to find out if twinning is symmetrical or asymmetrical and in the absence of scientific proof we need to be agnostic as to whether a human embryo is a human being or not. Why the change of tactic, if it is the role of science to determine if an embryo should be left alone or killed, what scientific evidence can you offer in defence of your right to live, or more specifically your right not to be killed (none).

    So no there is no need for agnosticism nor do we need to hang out for a science discovery. This whole debate is the result of the expression of intent to kill. How to pull it off is the only question.

  • bethyada:
    “I am saying that although identical twins start with the same genetic information, some genetic changes may take place….”
    Which is why even “identical” twins have the odd difference by which they can be distinguished. But I fail to see what relevance that has to this discussion.

    cj_nza:
    Well said! It is too easy to focus on the details and miss the big picture.

  • It could be that I’ve missed something here, Matt, but all I can see is you arguing for agnosticism over whether or not the conceived individual continues to exist in the case of twinning. But this is not at all the same as showing that we should be agnostic about the status of the conceived entity prior to twinning. Why not just say that the conceptus is fully human in any morally relevant sense, and – like an adult in some strange brain splitting experiment – we should remain agnostic about whether that same human continues after twinning as occurred, but we have no reason at all to doubt the fully human status of the embryo prior to twinning? Physically, one of the products of twinning is almost identical to the embryo prior to twinning.

  • Re-reading through the comments section, I see that perhaps you were only arguing, Matt, that we should be agnostic about when the persons after twinning began to exist. With this I agree, and I’m assuming that you will agree that this does not lead to agnosticism about the status of the embryo prior to twinning.

  • Here’s my two cents worth, for what it’s worth…

    A. I like the analogy of the flatworm used by Robert George and Christopher Tollefsen – parts of a flatworm have the potential to become new flatworms when they are separated from the whole flatworm they are currently part of, but this doesn’t mean that the current flatworm is not a unitary individual

    B. Another spanner to throw into the machine… there may well be some sort of Siamese type arrangement that goes on before defined twins can actually be observed by science – where there are two presences from the beginning who share the same body mass for the early stage of their development

    C. If you are one of the vast majority of embryos that doesn’t twin, then what were you from the point of conception until the appearance of your primitive streak? This is actually quite an important question, because if you want to argue that you did not come into existence until the primitive streak is observed, then you need to explain what the entity was that existed prior to (approximately) day 14, and why it acted the way it did

    D. Right up until the point of the appearance of the primitive streak, the early embryo does not merely act as a group of merely adhering cells, instead it acts as a unified entity.

    To quote from George and Tollefsen:

    “…the early embryo has many of the obvious goals of a whole organism, and it undertakes directed activity in the service of those goals”

    They identify at least three clear goals, which the early embryo (prior to the primitive streak appearing) purses n such a way as to indicate that it is a unified whole, and not merely adhering cells.

    The 3 goals are:

    1. Getting to the uterus
    2. Develop the structures needed to make implantation possible
    3. Preserve its structural unity against various threats

  • Hi

    First, I am a little puzzled by your suggestion that *I* “ intend to kill others”I have argued against killing embryos and am opposed to it. Moreover the whole argument is about whether an embryo is another in the relevant sense so this really begs the question.

    But to your comments,

    First you state is. “A human embryo is likely to be born if you leave it alone. It lives and is human” this argument assumes that the organism which is the embryo will latter be born if left alone. That follows only the embryo and the born infant are latter stages of the same organism, the whole argument re twinning and other issues is precisely whether this is the case.

    Second you ask “if not a human being what other type of being does it purports to be?” I am not sure this argument words, supporters of abortion ( which I am not) have given an answer a genetically human organism that is not a person, I don’t agree but that’s a perfectly coherent answer. One could similarly make similar arguments about human. Cancer cells these are beings, and have human genetic code so one could ask, if they are not human beings what are they? Obviously the issues are a bit more complex than this.

    Finally I have never said it is the role of science to determine whether its OK to kill. My point is that science does provide us with information about pre-natal development that is relevant ( but not by itself sufficient) to answering these questions. There is nothing odd about this. The fact provided by Physics that nuclear explosions are likely to cause massive civilian causalities is relevant to deciding whether to use them. The fact Provided by the science of ballistics that bullets damage peoples brains is relevant to deciding whether to shoot someone etc. In ethics we have moral principles such as do not kill human beings without justification, principles about what constitute justifications and also factual issues such as what things are human beings and what situations in fact fall under the descriptions laid down in the justification. Science is relevant to these latter factual issues.

  • Hi Mr Dennis

    Thanks for your comments allow me to explain why I am not completely convinced by this sort of reasoning.

    1. Start with a null hypothesis. This is the simplest option – ie that human life and personhood are the same thing and start at conception.

    The problem is this is false, God is a person and God is not a human being, hence personhood and being human being are not the same thing. Presumably if it was discovered that intelligent extra terrestials existed they would be people and not human beings.

    2.So the soul is irrelevant to the current debate, as we know the embryo is (genetically) a human body that can be killed. So is a corpse it’s genetically a human body and it cannot be killed. So this doesn’t work either.

    Issues such as twinning tell us that in the case of identical twins its doubtful their bodies began at conception, and similar issues such as cell splitting in the early stages cast similar worries.

    3. e is no other point during development when the embryo makes a major change such as that at conception, development after that is gradual and linear.

    I would call splitting into two a major change and one that arguably precludes linear development.

    4. twinning argument does not present any evidence for the alternative hypothesis, just adds a little bit of interesting confusion to the issue for ONLY those few babies that happen to twin – which is rare, how many identical twins do you know? It is a minor side-issue that is interesting to discuss but ultimately irrelevant, especially as it is inconclusive in either direction.

    Don’t know why I need to keep saying this but I agree that the twinning argument only applies to twins. However as I noted there are other arguments involving cell splitting which are similar and apply to all embryos these cannot be side stepped so easily. I note that no one has addressed these. I would love to see them refuted but ignoring them and making bad arguments for the other side is not a rebuttal.

  • matt, forgive me if I’m simply having the reading equivalent of being “hard of hearing,” but I still cannot locate the argument you’re referring to. You say, referring to the fact of twinning, “Many Theologians and Philosophers have argued that this fact provides strong evidence that human beings do not come into existence prior to 14 days post conception.” You then outline that argument. But that argument is only an argument for agnosticism about whether the pre-twinning embryo survives the twinning process.

    I don’t see how this translates into the conclusion that you say “many philosophers and theologians” are drawing. If the possibility of brain splitting in adults does not lead to agnosticism about whether pre-splitting adults are human beings, then why should the argument work for embryos?

    How is there not an obvious prima facie case for saying that since pre-twinning embryos are genetically human organisms, they have human parents, and they have e human future (provided we accept that splitting is a hypothetical possible future for an adult), I see no grounds for agnosticism. Agnosticism about whether there is a post twinning embryo that is numerically identical tot he pre-twinning embryo, sure. But agnosticism about whether that pre-twinning embryo is a human being? Why?

  • PS (I wish I could edit existing posts of mine), I’m talking about this comment, Matt: “there are other arguments involving cell splitting which are similar and apply to all embryos these cannot be side stepped so easily.”

    What are those arguments?

  • With the luxury of cut and paste some of your sentences, hopefully none out of context:

    “My agnosticism is based on three things. The first is that I know of no reason for thinking an embryo is a human being at the time of conception.

    I argued that in cases where we do not know if an embryo is a human being it would be reckless and hence, gravely wrong to kill it because it might be.

    So in conclusion, I think that the twinning argument should give us pause about claiming that human beings come into existence at conception. We do not know whether twinning is symmetric or asymmetric. If it is the latter, we do not know whether anyone who is a monozygotic twin came into existence or conception or not.”

    I did not intend to attack you personally, must be an artifact of a mannerism from a previous culture/language/life.

    My point of view is that in the absence of proof you can choose to be agnostic or you can give any of the competing arguments the benefit of doubt as a stated position.

    I find it by no means contradictory to suggest that an embryo found in a human womb is in fact a human being. Thus to use the foundation of your sentence, I can think of no reason for thinking that an embryo is not a human being from the time of conception.

    The arguments around twinning and our uncertainty on symmetry is not contradictory to that view at all. As stated in another comment it would be reasonable to assert that an identity exist and that a second identity could come into being through the process of twinning (asymmetrical) or that we need to remain agnostic as to the identity of either of the products of twinning(symmetrical).

    As indicated before, I did not suggest that you are a proponent of abortion but; and here I believe is where I roads diverge, you oppose abortion because “you might be wrong in believing that the embryo is not human”, whilst I appose abortion because terminating(killing) a human being (embryo) is wrong.

  • Hi cj-nza

    You write:
    My point of view is that in the absence of proof you can choose to be agnostic or you can give any of the competing arguments the benefit of doubt as a stated position.
    This strike me as an incoherent position, when I talked about “absence of proof” I was clear that I mean’t absence of arguments either for or against the proposition that embryos are human beings. To say that in the absence of proof one should give ‘benefit’ of the doubt to competing arguments is absurd. If there is an absence of proof then there are no good arguments for this proposition, to infer from this that one should assume the arguments are good is to contradict oneself.
    I find it by no means contradictory to suggest that an embryo found in a human womb is in fact a human being. Thus to use the foundation of your sentence, I can think of no reason for thinking that an embryo is not a human being from the time of conception.
    This is problematic on two counts, first you state there is no contradiction in affirming that an embryo is human, agreed, there is also no contradiction in denying it either so this proves nothing. Second, the fact that there are no good reasons for denying a proposition does not mean that there are good reasons for affirming it.
    As stated in another comment it would be reasonable to assert that an identity exist and that a second identity could come into being through the process of twinning (asymmetrical) or that we need to remain agnostic as to the identity of either of the products of twinning(symmetrical).
    Yes and I have responded to this comment more than once, I never said the twinning argument gave us reason to be agnostic about wether all embryos are human prior to conception. I said this argument shows that we don’t know that identical twins come into existence at conception. As I repeatedly stated my agnosticism is based on other grounds, notably the absence of any argument either for or against the claim that embryos are human beings.
    here I believe is where I roads diverge, you oppose abortion because “you might be wrong in believing that the embryo is not human”, whilst I appose abortion because terminating(killing) a human being (embryo) is wrong.
    Actually I oppose abortion because I consider it homicide, as I stated I believe there are good reasons for thinking a fetus is a human being, my agnosticism was with regards to the embryo. Abortions are typically done on fetuses not embryos. Moreover, if the twinning argument was sucessful it would show only that embryos are not human prior to 14 days post conception, thats quite compatible with opposing abortion as homicide, in fact many defenders of the twinning argument are strong opponents of abortion. Paul Ramsey is an example, Philip Devine is another, Don Marquis holds a view a bit like this. So your assesment is mistaken.

  • Brendan and Glenn

    To repeat myself yet again, I do not accept that the twinning argument provides grounds for holding that we should be agnostic about all embryo’s.

    It would be nice if people here stopped attributing to me an argument I never made and I never claimed to make and have repeatedly stated I am not making.

  • Matt, you started out bys aying “Many Theologians and Philosophers have argued that this fact provides strong evidence that human beings do not come into existence prior to 14 days post conception.” You also closed by saying that “So in conclusion, I think that the twinning argument should give us pause about claiming that human beings come into existence at conception,” and your recently said that “there are other arguments involving cell splitting which are similar and apply to all embryos these cannot be side stepped so easily.”

    Forgive me for thinking that this suggests a belief that we should be agnostic about whether humans come into being at conception, and the belief that there are other arguments (which I asked about in my last post) that apply this agnosticism to all embryos. These are, after all, the two things I mentioned.

    I acknowledged that you said (in the discussion that followed your ambiguous opening post) that the twinning argument, as far as you accepted it, only applied to the identity of the resultant twin. But you’ve also said of pre-twinning embryos that “I know of no reason for thinking an embryo is a human being at the time of conception.” I gave a couple of reasons, which you didn’t say anything about. But it really looks like you do think there are such “other arguments” that do apply to pre-twinning embryos. What are they?

  • Points taken, nothing further to attribute.

  • Hi Glenn,

    The argument I refer to comes from Don Marquis ( who for those apart from yourself who may read this, Marquis is a very strong opponent of abortion) Marquis after conception a single celled zygote ( lets call it Z) splits into two cells. (X and Y). He then notes that this creates problems about identifying which of these two new cells is identical with the preceeding one. Any reasons for identifying X with Z will also provide reasons for identifying Y with Z, hence the appears no rational way of answering this. If one states that its both Y and Z individually the transivity of identity leads to contradictions. If one identifies it with the collective of X and Y together questions arise as to why given that the individual cell X is no different from Z and one stated Z was a human beings, one does not identify X and Y as two new human beings as opposed to parts of a singe one. The only reason I can think of is to preserve the belief that life begins at conception.
    The two cells split into 4 4 into 8 8 into 16 and so on and the problems continue to arise until cell differentation occurs and its obvious that we have cells that are part of one unitary organism.

  • Matt

    The reason this interests me at present is actually because Marquis’ argument against the claim that human beings (non-twinned, to avoid that side track for now) began at conception has relevance for arguments in philosophy of mind,a series I’m working on at the moment.

    My instinctive answer to the question of “which cell is identical to Z” is to say that neither of them are, and that this doesn’t matter. Every cell in the body ceases to exist, to the benefit of future cells in the body. If cell z was replace with x and y then I can see why it might be a problem, but what happens here is that cell X actually becomes X and and Y.

    Approaching it in the way Marquis does is a bit like saying that since X and Y were not conceived, they are not the same entity that was once entirely contained in Z. In other words, only by assuming that it’s not the same organism can this objection seem compelling.

    Another angle to take is that it’s only accidentally true that prior to cell division, one cell was a human being. That’s why it’s not problematic to say that “the collective of X and Y together,” while not identical with the cell (that is, they are not qualitatively identical with it – as a collective they do not share its accidental features), are identical with the entity that was made up by that cell. Reasoning this way, nobody can object that:

    If one identifies it with the collective of X and Y together questions arise as to why given that the individual cell X is no different from Z and one stated Z was a human beings, one does not identify X and Y as two new human beings as opposed to parts of a singe one.

    I do not say that X and Y are two new human beings, because I would not have said that the property of being a one celled zygote is identical with the property of being a human being. All I would say is that the two-celled thing is numerically identical with the previous one celled thing.

    I’ve downloaded Marquis’s paper (“The Moral Principle Objection to Human Embryonic Stem Cell Research”), and in light of our discussion here – and also the material I’ll be working on in philosophy of mind – I’ll write something for the blogosphere.

  • For those interested in seeing what Glenn came up with it is on his blog here Don Marquis on Embryonic Stem Cell Research.

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