Me and My Zygote

Bill Vallicella of Maverick Philosopher has asked for assessments of an argument by Barry Smith and Berit Brogaard to the effect that zygotes are not substantially the same as the successor entitites that they (apparently) develop into. Bill summarizes the argument as follows:

1. The unicellular zygote is predestined to undergo fission.
2. Whatever undergoes fission ceases to exist at the moment of fission.
3. The unicellular zygote will cease to exist at the moment of fission.
4. If a substance S ceases to exist at time t, then no substance S* existing at a time later than t is transtemporally identical to S.
5. The unicellular zygote is a substance.
6. A post-birth human being is a substance.
7. No post-birth human being is transtemporally identical to a unicellular zygote.

In what follows, I will use the term “zygote” as shorthand for “unicellular zygote”, with exceptions to this protocol being somewhat obvious.

Bill closes by asking “Can someone tell me what is wrong with this argument?” Since a zygote is a conceptus (that is, a fusion of a male haplocyte with a female haplocyte), this argument should be of some interest to Catholics. If I am not transtemporally identical to any zygote, and no zygote is transtemporally identical to any (more fully developed) human being, does it follow that zygotes are not the same sort of substance as a (fully developed) human being? If they are not, and if humanity constitutes a natural kind, it would seem that zygotes are not humans at all and, hence, there is no reason to protect them from destruction by abortion.

Where, if anywhere, does this argument go wrong? It is worth noting right from the start that certain elements of the argument are left rather vague. What does it mean to be a substance, according to this argument? What does it mean to be a human being? Or a zygote? What does it mean to insist, as premise (2) does, that the zygote ceases to exist when it undergoes fission?

The argument appears to presuppose a rather strange sort of materialism, one in which substances both are, and are not, mere heaps. For example, it appears that we are to think of zygotes as being single-celled entities, irreducibly. That is, a zygote is not merely a heap of molecules of a certain kind arranged in a certain way, but rather the zygotishness of the zygote emerges from that heap. I say this because the argument insists that the zygote, qua instance of its kind, ceases to exist when it ceases to be a single-celled entity. In other words, the being of the zygote appears to be regarded as dependent upon the single-celledness of the thing, not upon the thing being constituted of these or those molecules.

Fission, in this argument, appears to be a term of art for the sort of division that zygotes undergo–it is not merely a division of any material being into parts, since it is obviously not the case that I cease to exist when I lose a finger or a toe in an accident (I say “obviously” even though there are those who would disagree). On the one hand, if it is a term that applies only to zygotes, then it appears to beg the question; on the other hand, if it is a term that applies to any kind of cellular division, it is not clear why the substancehood of the zygote is affected by it when, for example, my own substancehood is not affected by the fact that my own cells are undergoing division all the time. In short, the fully developed human being is also a heap of a certain kind: a heap of cells. The argument does not discuss any sort of constitutive relations with regard to substances: are substances metaphysical unities, or are they constituted of certain kinds of parts? If the latter, what are the mereological relations between the parts and the whole substance? None of this is made clear by the argument as it stands.

If I am reducible to merely physical parts, and if substancehood depends upon being constituted by these physical parts in particular, then it appears that I cease to exist every seven to twelve years, since on average a human body comes to consist of entirely different material parts within that period of time. Do I become a new substance at that time, or am I merely a new instantiation of the same kind of substance? In either case, what on earth is the motivation for adopting such a view, which appears to be so at odds with our subjective experience?

The second premise appears to be the most problematic, since it is hopelessly vague about what it might mean to exist as something. If a zygote literally ceases to exist after fission, that can only be because we are using the name “zygote” to pick out the heap of molecules as they were arranged prior to fission; after fission you have roughly the same bits of physical stuff, but they are arranged in a new way. The second premise asserts that we can no longer use the word “zygote” to pick out the heap of molecules after fission, but does it follow from this that we have an entirely new substance on our hands after fission? If the definition is merely a nominal one, this does not follow at all; if it is a real one, then the argument begs the question. Premises (5) and (6) assert the substancehood of both the zygote and the “post-birth human” being, but they rely on the sloppy mereology of premise (2) to get the conclusion, (7), which is also rather sloppy since what it appears to mean is nothing more than that zygotes and post-birth humans are different kinds of substances, since it seems rather obvious that they are not straightforwardly identical in some Leibnizian sense.

Let us grant that zygotes cease to exist at the moment of fission in the sense that the name “zygote” is no longer the correct name for what is present after fission. This much is merely elementary biology. It is also an accident of the history of elementary biology that we just don’t happen to have different names for the subsequent masses of cells–names that correspond to the number of cells present. A zygote is, by definition, a unicellular thing; after fission it becomes an embryo, and that is what the thing continues to be until, I supppose, it magically turns into a “post-birth human” simply by slipping through the vaginal canal. Much difficulty could be avoided, I suppose, by remembering that the term “fetus” applies to every stage of development from conception on, including zygocyte. The word “zygote” is derived from a Greek term meaning “joined”, and it really refers only to the joining of male and female haplocytes–I don’t think it was ever intended to have the metaphysical ramifications that Smith and Brogaard appear to want to endow it with. If we grant their premises, which appear to have been crafted for the sole purpose of proving that “post-birth humans” are not the same kind of thing as zygotes, then we will be forced to say that, from conception on, there are really three things involved: the zygote, which ceases to exist fairly early on; then, at some point much later, there comes the “post-birth human”; and covering the entire temporal frame there is the fetus, which is substantially identical with both the zygote and the “post-birth human” even though the zygote and the “post-birth human” are not identical with each other. In other words, the argument forces us to invent a new form of identity that is not transitive.

To avoid this mess, I suggest we jettison premise (2). Without premise (2) the argument fails, but at least we are spared the effort of writing another whole philosophical paper on the nature of non-transitive identity. Sadly for Smith and Brogaard, we are also spared the conclusion that we apparently wanted to draw, namely that zygotes are not the same sorts of things as “post-birth humans”. I suppose if we wanted an ontological defense of the licitness of abortion we would be really disappointed by now.


22 Responses

  1. Scott:

    Needless to say, I’m very interested in finding ways to undermine the Smith-Brogaard argument (‘SBA’ for short), and I like the way you go about it. However, I think they might salvage what they’re after with (2) by reformulating that premise.

    As it stands, (2) is too vague because it doesn’t distinguish between ceasing to exist as a substance and ceasing to exist as a stage in the development of a substance. It’s true if taken in the latter sense, but unhelpful to SBA; it’s false if taken in the former sense, even though it would be helpful to SBA if true in that sense.

    Perhaps they can’t come up with a way to salvage (2) and reach the result they want. But I’ll bet my clunker that somebody over at Maverick Philosopher will try.


  2. Mike

    I agree that (2) could be reformulated in a variety of ways, but it seems to me that the variety can be boiled down to two possible upshots. On the one hand, the premise could be made to work towards the intermediary conclusion, (3), by making explicit certain essentialist assumptions (this, I take it, is what “n–blog” tries to do in his comments at MP). The full SBA article, linked to at MP, contains an explication of the relevant notion of substance that they want to underwrite the argument, and it is largely a causal notion with clear Aristotelian and Cartesian influences. (Indeed, it seems to me to be something of a mishmash, but Barry Smith is an excellent ontologist.) On this way of proceeding I do not see how the argument can avoid begging the question, since the second premise is basically asserting what the main argument is supposed to be arguing for (that is, it asserts a version of [7]).

    On the other hand, the second premise could be taken to be claiming nothing more than that what is present after fission is no longer a zygote. But this is clearly far too weak (because it is so vague) to support the rest of the argument and, indeed, seems largely irrelevant to (7).

    I may be wrong, but it looks to me as though other possible readings basically boil down to this choice between essentialist / non-essentialist approaches. I’m sure you’re right, though, that somebody at MP will defend SBA (Bill himself argued for accepting [2] in an earlier post).


  3. If I may drop a thought in with two better philosophers than I, I’m not sure (2) is the weak point here. I have to wonder whether the bigger problem with SBA is the issue of identity, not so much in (2) as later in development. An argument that the “I” of today is ontologically different from the “I” of yesterday will gain no traction, however cleverly it may be constructed; our self-understandings as continuing beings are too insistent to let such an argument pass the smell test. Yet that very discontinuity, not only before birth but after birth, seems implicit in SBA’s insistence on material identity (including organization, as Scott notes) as necessary for ontological identity.

    SBA might certainly reply that, since not all my cells undergo fission at once, there is still an “I” that exists transtemporally after birth. Such a concession would make SBA all but irrelevant to abortion, however, since it would concede that at any stage after the first cell division, the entity that is the fetus is transtemporally identical to the entity that is the baby. Given that the first cell division happens 12-24 hours after fertilization and that thereafter the transtemporal identity is undisturbed, even an abortifacient that acts by preventing implantation would thus be conceded to be killing a human being.

    So it would seem that SBA is either patently untrue or all-but-irrelevant to the abortion debate.

    Now y’all can tell me which herd of elephants in the room I missed :-).


  4. Perhaps I’m being dense, because I am having trouble figuring out what’s so troubling about this argument even if it’s sound. Are we supposed to conclude that the zygote is not entitled to rights because it doesn’t possess transtemporal identity? Does the conclusion serve as an important premise in another argument? Or is there something more subtle (or obvious) that I am missing?

    Assume that the argument is sound, and so the one-celled zygote is transtemporally different than a two celled-zygote. The basic questions concerning abortion still remain: are both the one and two celled zygotes alive? Human? Entitled to rights?

  5. I think Peter and Paul (nice juxtaposition) are onto something but not quite nailing it.

    As I ponder further, it seems to me that the real problem with SBA is its systematic ambiguity about ‘substance’. If we suppose, plausibly, that the unicellular zygote (UZ) is identical with a certain substance S, the fission of UZ does not cause the substance with which UZ is identical to cease to exist. It only causes S to cease to exist as UZ. That makes SBA an enthymeme at the very least. Its logical validity requires the premise, here only tacit, that whatever does not remain transtemporally identical with S with respect to the matter comprising S cannot be the same substance as S. But in that case, I would be a different substance at least every seven years, because it is over that period that the atoms comprised by my body are completely replaced by others.

    I guess what all this shows is that debates about the ontic status of the zygote aren’t terribly useful without prior agreement on the content of our fundamental ontological concepts. Good luck getting that agreement.

  6. 1. The unicellular zygote is predestined to undergo fission.
    2. Whatever undergoes fission ceases to exist at the moment of fission.
    3. The unicellular zygote will cease to exist at the moment of fission.
    4. If a substance S ceases to exist at time t, then no substance S* existing at a time later than t is transtemporally identical to S.
    5. The unicellular zygote is a substance.
    6. A post-birth human being is a substance.
    7. No post-birth human being is transtemporally identical to a unicellular zygote.

    Is this valid? Let A be a zygote. Suppose A does cease to exist through fission and produces B and C. What’s the problem? It just means that B and C did not come into existence from conception, but rather through fission of A.

    Okay, now, suppose we say that D is a zygote and did not go through fission. D becomes D*, that is, an infant. I fail to see why D* is not transtemporally identical to D, the unicellular. So even if the case of A bringing about B and C is true (1-6), it doesn’t mean 7 is true.

    This just means: not *all* human life comes into being from conception (ex. B and C). This does not mean that A does not have any value, of course.

  7. A little addition to my last statement. So, not all human life comes into being from conception but it does not follow that not all human life that comes into being from conception does not have a right to life. A human being, no matter when and how it came from, has the right to life.

  8. I must admit I am also initially puzzled by the alleged cogency of this argument. (2) is what jumps right out at me as problematic. I am confused how the identity of a “genetically coherent” person (i.e., the person who “bears” their own genome from conception to death) is compromised by a transition from unicellularity to multicellularity. What does the number of cells have to do with the substantial identity of the person in which and as which they exist? The whole process of fission is meant to grow a genetically coherent person based on the same genome. Genetics is not chemistry; adding a second hydrogen atom to oxygen makes a new molecule, but adding a second cell to the zygote does not make a new zygote.

    I suppose the point is that, if a person is at least a unicellular zygote, then the fission of that zygote is the destruction of that zygotic person. I need to review the argumentation again, of course, but it seems completely tautological to say a unicellular zygote is destroyed by becoming bicellular, since by definition a UNICELLULAR zygote can only be unicellular.

    It looks like a simple case of perdurantism, applied to biology, where the unicellular Person is just one “gene slice” in adjacent to a bicellular, tricellular, quadricellular, etc. Person slice.

    I recall Fr. Tacelli had an essay titled “You were a zygote.” Seems highly pertinent here.

  9. My own intuitions are with those who think that this argument is muddled about substance. The paper from which the argument was abstracted has a more detailed account to give of substance, but even there it does not fully explain why we should accept the upshot of premises (5) and (6), that is, we are not told why a zygote, qua zygote, is to be thought of as a different substance than a “post-birth human” qua “post-birth human”. As I suggest in my post, the term “zygote” is just a name for the unicellular version of a human being; there is no compelling reason to think that some particular substance ceases to exist just because the human ceases to be unicellular in constitution, any more than there is a compelling reason to think that, when my seven-year-old body grows many more new cells, something called a “seven-year-old-human” ceases to exist only to be replaced by a “post-seven-year-old-human”. Ontologically that is one possible reading of the situation, of course, but it seems rather strained–almost Scholastic in its approach.

    So unless the paper can give a better account of constitution, it seems to me to fail to give a coherent reason for thinking that there is anything metaphysically interesting about being a zygote as such vis-a-vis being a human being as such.

  10. Scott,

    Maybe the distinction between substance and person is appropriate here. I think what Brogaard-Smith proves is that the individual at t may be different from t*. I think I can agree with that. However, it does not mean that the substance changes. So, A produces B and C. B and C are different individuals but they keep the same substance that is, that of a rational nature.

  11. Apolonio,

    I’m puzzled. You wrote: “suppose we say that D is a zygote and did not go through fission. D becomes D*, that is, an infant.”

    First, assuming the fist premise of the argument, how could it be that D did not go through fission?

    Second, how the zygote D become an infant without fission?

    Apolonio and phamilton,

    Analytically, the question of the right to life seems to be different from the given subject (i.e., SBA). Cf. We should distinguish (A) post-birth humans are transtemporally identical to the respective zygotes and (B) it is always morally wrong to intentionally kill some human zygote (at least least in case of common abortion motives). (By saying “human zygote”, I do not wish to assume that human zygotes are humans.) I suspect there’s an extensive, raging philosophical dispute whether (A) entails (B). E.g., what if it had been morally permissible to deliberately kill me when I was a zygote (assuming that I really was) because I gained my right to life later, by way of gaining some relevant biological or psychological feature? I am a Catholic, I do not hold this view, but we should handle such questions with care.

    Fourth, though (A) and (B) seem distinct, (A) seems relevant to (B), too. More precisely, (the sufficient probability of) (A) — together with claims like (C) no loosing or gaining of (non-essential) properties is ever morally/axiologically relevant in the case of entities transtemporally identical with post-birth humans — constitutes the main reason for (B), doesn’t it? Of course, there could be other reasons. E.g., the teaching of the Church that abortion is wrong, regardless of the issue of transtemporal non/identity of zygotes with post-birth humans and regardless of the question whether zygotes are humans (even if they are transtemporally non-permanent and perishing after short intervals of life). But this apologetical reason seems much more controversial than the embryontological, to say the least.

    Fifth, if the premise (2) in SBA is true, then what? Well, then, it seems to me, we have a series — though I don’t know how many members it has — of perishing cellular organisms. Thus, before the birth of a child there is a long series of corresponding, perished, preceding twins in the womb of the mother. To paraphrase the commenter Paul at BV’s blog: if “the arguments for life beginning at conception were true, and if a genetically identical human had the same parents as you but wasn’t you, then it would be your twin, then we could conclude that: all post-birth humans have had twins (note the plural) that died in the womb. … [which is] something highly counter-intuitive.”

    So far, the comment by Elliot B seems to me as the most illuminating reaction to SBA made here. I’ve made further suggestions at BV’s blog.

  12. Apolonio,

    How could it be that B and C are “different indivuduals,” but also “the same substance”? Do you mean that B and C are numerically distinct, but of the same (natural) kind?

  13. Vlastimil,

    Ah, for some reason I was thinking of twinning. I thought that issue is the one that is more challenging.

    And I meant same nature, not substance. I got them mixed up.

  14. Vlastimil,

    So D does not go through twinning. That’s what I meant from D to D*.

  15. Apolonio

    You may be right that some distinction between individuals and kinds is required, but I myself would shy away from the notion of personhood in this context. I think biological categories will probably suffice. And Vlastimil is right to point out that, technically speaking, it is possible to separate the question of right to life from the metaphysics of substance continuity, so categories like “person” may well be superfluous anyway.

    The issue, it seems to me, that connects the metaphysics of the paper with the question of rights, is the issue of constituency, that is, why ought we to regard the cell as the basic constitutive element of prime metaphysical importance, as SBA appears to do, rather than some other material element or conglomeration of elements. It seems to me to be totally irrelevant that a unicellular entity ceases to be present in the womb after the fission of the zygote. True enough, a zygote ceases to be present, but a lot of things could also be said to cease to be present, and none of these things is obviously relevant to the question of whether what is present at conception is transtemporally identical to the “post-birth human” that we are said to have at the end. What seems to me to be most relevant is that, from conception through natural death, we have roughly the same DNA present, but S&B never address this in their paper.

  16. Scott,

    I don’t think biological categories will suffice when it comes to the moral sphere. Of course the issue here is metaphysics, but I think it is safe to say that this metaphysical argument is made so that it can impact the moral status of embryos. That’s what I was after. I know that’s not really part of the argument, but that’s where my head went when I read the argument.

  17. I would like to add that even if we grant SBA, despite the wobbliness of certain of its premisses, I fail to see how it cuts against classical hylomorphism.

    First of all, what is it that “predestines” said zygote to split? It is the genetic “form” at work within the zygotic material. Undergirding the “genetic form”, it is the soul, as a divinely infused principle of dynamic order (form), that gives coherence to the genome FOR the subsequent and ongoing development of the person to whom the genome belongs. Unless SBA is prepared to say the products of fission are themselves individual human “substances” (only now ones that do not undergo fission), it is utterly ignoring the formal coherence implied in speaking of the subsequent bicellular fetus as products OF the zygote. They are obviously related (and by predestination , no less!), but how do we account for their relationship? By admitting the formal unity that underlies them.

    Second, looking to a simple material change (from uni- to bicellularity) to show a formal (substantial) change completely misunderstands the “ArisThomistic” relation between form and matter. While there are certainly material limits to the formal power of substance to actually exist (e.g., there are no bronze humans, no atoms made of cheese, etc.), this is no way undercuts the key premise of Catholic hylomorphism, namely that it is precisely the divine will that the rational soul (anthropic form) develop materially within certain limits––one such limit being the fission from zygote all the way to baby on bosom. Indeed, I would say SBA actually (unwittingly) gives away the game in its first premise, or at least plays right into a Catholic objector’s hands: it is precisely the NATURAL, formal inevitability of the zygote to undergo fission AS PART OF HUMAN-FETAL DEVELOPMENT that grounds the defense of human life all the way back to the zygote!

    What’s wrong with SBA? It tries to encase hylomorphism in mereological materialism––clumpism––and thus distorts the relevant terms in its favor. It seems as grotesque a handling of hylomorphism as saying that, if we cut a written page in half, it ceases to bear the meaning on the paper (as if half a sheet of Shakespeare were no still formally Shakespeare).

  18. Apolonio

    The reason why I think that biological categories will suffice is because I think they are all that we have available in a philosophical argument of this kind. If you are dealing with naturalists, you have to make a naturalistic argument, and the convenient thing about the biological categories, at least as far as I can see, is that they support, rather than work against, arguments in favor of thinking of there being an ontological continuity of some kind from the moment of conception to the moment of natural demise. Whether the same sort of end can be achieved with non-naturalistic arguments is, I think, an interesting question, but in this particular case it may be best to work with a certain audience in mind.

    If it can be shown that there is an ontological continuity from conception to death, then we have already done the naturalistic footwork of showing that there is no substantive difference between a zygote and a “post-birth human”. Granted, this does no work from a moral point of view, but then showing that zygotes are different in kind from post-birth humans does not entail any moral conclusions, either. The problem, as I see it, is that naturalists want to make use of what they think of as “scientific” results to support philosophical arguments, and this is a case in point. If you can show that the scientific argument actually supports a different sort of philosophical argument altogether, then I think you’ve gone a little way towards showing how a different moral argument can be drawn from the same empirical data.

  19. Scott,

    You wrote: “… showing that zygotes are different in kind from post-birth humans does not entail any moral conclusions …”

    Let me a question. If human zygotes were different in kind from post-birth humans, which arguments for the wickedness of intentional killing of such zygotes could you construe?

  20. Vlastimil,

    I didn’t express myself very well in that comment. What I meant was that, supposing you are out to show either that zygotes are deserving of protection in the womb or that they are not deserving of protection, merely showing that they are different in kind from “post-birth humans” would not be a sufficient condition for proving either hypothesis.

    Unless of course one believes that human beings, as such, are always deserving of protection. In that case, showing that a zygote is the same in kind as a human would suffice to show that it is deserving of protection, given that assumption. If you think that the zygote is different in kind from human beings, simply being different in kind would not suffice to show that zygotes are not deserving of protection. Similar, if one supposes that human beings do not necessarily deserve protection from killing simply by virtue of being human beings, showing that zygotes are either the same in kind or different in kind would not suffice to show that they either are or are not deserving of protection.

    My own view happens to be that human beings, if they have any right to life at all, they have it simply by virtue of being the kind of thing that they are, that is, in virtue of being a human being. My reason for thinking this is the fact that there are no other, non-arbitrary reasons for positing a right to life for human beings (for example, attaching the right to something called “personhood”, or “sentience”, or the various other tricks used to support abortion while denying the licitness of killing “post-birth humans”). On my view, then, establishing that the zygote is merely a human being at a certain unicellular stage of development is sufficient to show that zygotes deserve protection from wrongful killing. My reason for thinking that they are the same in kind as “post-birth humans”–that they have roughly the same DNA and that DNA is determinative of biological kinds–is also sufficient to rule out even the logical possibility that they are not the same in kind as “post-birth humans”, which is rather convenient, if I do say so myself.

  21. (2) follows only if we assume that physical entities (such as zygotes) endure – that is, that a physical entity E exists in its entirety at any instant T of E’s history by having all its parts at T. If, however, we allow that E perdures – that is, that it persists through time by having a temporal part at every instant of its history – then there is no problem with E having a temporal part P1 at T which is unicellular and a temporal part Pn at T+n which is multicellular.

    On perdurantism, (4) holds – Pn is not transtemporally identical to P, as they are distinct, different parts of E.

    However, on perdurantism, while (7) is true, it is only true in the sense that a unicellular zygote is not a multicellular organism (which seems entirely self-evident). However, as both P and Pn are parts of E, they are parts of the same ‘substance’ (if I understand the notion of ‘substance’ aright) – shall we say that, on perdurance, they are both part of a physical entity which is extended in both space and time.

    However, some may find that perdurance comes at too high a price (and there are few Christian perdurantists) : if one holds that physical entities perdure, one has either to accept eternalism (the view that ‘other times are as real as other places’) or to accept that physical entities can have as parts things that do not exist.

  22. Hello,
    I am new to the discussion and to your blog. The arguments above are very interesting. I have a few points to pose.
    Premise 2 seems strange to me for three reasons.

    1. In biology, the development of organisms is a natural function of their existence. In order to appear as a mature organism, said organism goes through a series of stages to reach maturity. Therefore, in all those stages, it is the organism that it’s DNA programs it to be.

    2. There are concepts of active and passive potential. A heap of scrap metal has passive potential to become a car. An agent outside the boundary of the scrap metal must do work to create a car.
    A lit match and dry wood that are in physical contact have active potential to become a bonfire. In just letting the lit match and dry wood “do what they do”, so to speak, the bonfire naturally matures.
    The zygote, if left in the correct environment has active potential to become a fetus, baby, toddler, child, teenager, and adult. A skin cell has passive potential to become a zygote.

    3. In terms of engineering, a car does not become a car because an internal stereo is added. The modified car still functions as the original car.

    (In general I like the SLED test.)

    Let me know what you think. (If I may: stop on by to my blog “Wondering Zygote Emeritus”.)

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