Here is yet another entry from the now-defunct Powerblogs site. It is pretty good, I think, and deserves to be kept online.
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Have I been in existence as one and the same human individual from conception on? Of course, I and any intra-uterine predecessors I may have had have been genetically human from conception on: at no time was there anything genetically lupine or bovine or canine or feline in my mother's womb. The question is whether I am numerically the same human individual as the individual that came into existence at 'my' conception.
The following argument seems to show that no zygote is a human being and that I have not been in existence as one and the same human individual from conception on. The argument is a variant of a much more complicated argument presented by Peter van Inwagen in Material Beings, Cornell UP, 1990, p. 152 ff. (In note 55, van Inwagen cites Peter Geach, The Virtues, Cambridge UP, 1977, p. 30.)
The argument is essentially this:
1. A zygote is already a human being. (assumption for reductio)
2. When a zygote divides, it ceases to exist. (premise)
Therefore
3. When a zygote divides, the human being it is ceases to exist. (from 1, 2)
4. At or after a zygotic division that terminates a human being, a new human being comes to exist. (premise)
Therefore
5. Pregnancy involves the creation of two human beings. (from 1, 4)
6. (5) is absurd: there is only ever one human being in the womb.
Therefore
7. (1) is false: A zygote is not a human being.
Since the inferences are valid, the soundness of the argument rides on the truth of its premises. I will not question the truth of (4). The normal outcome of (a human) pregnancy is the birth of a human being. Premise (2), however, seems open to doubt.
First we need to understand the reaoning behind (2). If Z splits into A and B, there appear to be three possibilities: Z continues to exist as A; Z continues to exist as B; Z ceases to exist. But any reason one gives why Z continues to exist as A is equally good as a reason why Z continues to exist as B. Since Z cannot continue to exist as two things, both of the first two possibilities are ruled out. This leaves the third: Z ceases to exist.
There is however a fourth possibility: when a zygote divides, it does not cease to exist, but changes from a one-celled to a two-celled organism. Of course, one thing cannot become two things. But a one-celled organism that becomes a two-celled organism is arguably one and the same organism which exists at two different times. One thing does not become two things; a one-celled thing becomes a two-celled thing.
Zygote Z becomes embryo AB. Must we say that Z ceases to exist and AB begins to exist? Why can't we say that the organism that is Z continues to exist as AB? Crude analogy: I have a burning log L in my fireplace. L breaks into two burning pieces P1 and P2. Does L cease to exist to be replaced by P1 and P2? One could say that, but it seems equally reasonable to say that L continues to exist composed of two distinct parts P1 and P2.
Van Inwagen rules out the possibility I am suggesting:
It does not follow, therefore, from the fact that the zygote is an organism, and hence a real object, that the two-cell embryo that replaces it is a real object. Why should we believe that there something that B and C compose? They adhere to each other, but we have seen that there is no reason to suppose that two objects compose anything. (Material Beings, p. 153)
I don't understand why van Inwagen says that "there is no reason to suppose that two objects compose anything." I find bizarre his denial that there are such things as ships and houses, and the implication above that an embryo, though composed of living things, is not itself a living thing.
Was I once a zygote? Yes, as far as I can see, van Inwagen's argument notwithstanding.
One thing is very clear: metaphysics is unavoidable. Just a little thought about a 'hot button' issue such as abortion lands you right in it.
See Van Inwagen on the Ship of Theseus and Puzzling Over van Inwagen's Denial of Artifacts