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.