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Conception and the Sorites Paradox

To recap, the problem of sorites arbitrariness is usually advanced by defenders of the conception threshold against so-called ‘developmental’, or post-conception, thresholds of personhood. As they see it, milestones like birth, viability, sentience, or brain development are all unacceptably arbitrary, since they equate personhood’s beginning with minor developments not at all morally distinguishable from those occurring moments before or after. Kaczor points out that the birth threshold in particular is not nearly as definitive as might be supposed. Is the crucial moment when the human being exits the uterus, or the vaginal canal? Need it be all the way out of the woman before it is born, and hence a person, or just some of the way out? Those who criticize developmental thresholds on this basis presumably believe that the conception threshold is immune from sorites arbitrariness. They must believe, that is, that conception is an entirely discrete event in the life of a new human, and one that is non-arbitrarily distinguishable from all of the developments leading up to and after it.

But proponents of the conception threshold would be mistaken in thinking that only post-conception thresholds are sorites-susceptible. Indeed, even the very description of conception as a discrete, identifiable ‘moment’ is considerably misleading. Here are some of the numerous, individual events comprising the conception ‘moment’:[1]

  • 1. The sperm approaches the unfertilized egg cell.
  • 2. The sperm passes the egg’s layer of follicle cells, and will make contact with the zona pellucida (the outer matrix of the egg) in one millionth of a second.
  • 3. The sperm makes contact with the zona pellucida (the outer matrix of the egg).
  • 4. One of the proteins in the egg’s outer layer binds to a molecule on the head of the sperm.
  • 5. The molecular interaction causes part of the sperm to release its contents, including enzymes that will enable it to penetrate the zona pellucida.
  • 6. Finger-like extensions of the egg cell take the whole sperm cell into the egg.
  • 7. The cell membranes of the egg and sperm fuse and break down.
  • 8. The nuclei of the egg and sperm form ‘pronuclei’.
  • 9. Membranes of the pronuclei break down and become arranged for ‘mitotic cell division’.
  • 10. The first division is the zygote, a single cell with 46 chromosomes, 23 from each pronucleus.

Of all these stages, which is the exact moment when personhood begins? Does the new person come into being the millisecond when the sperm first makes contact with the egg, or the next millisecond when it just begins to release enzymes and penetrate it, or during one of the milliseconds when the pronuclei are fusing? If penetration of the egg is the moment, how far must the sperm penetrate before a person exists, and why is any one of those microscopically distinct advances more significant than the adjacent ones? Even penetration by the sperm is not truly a ‘point’, but, at the quantum-mechanical level at least, is distributed over innumerable events. In truth, there are no ‘points’ of the non-sorites-susceptible sort anywhere in the biological world, if everything is only considered through microscopic enough a lens.

A proponent of the conception threshold of personhood who attacks later thresholds for being sorites-susceptible will need to show that his preferred benchmark is non-arbitrarily distinguishable from the adjacent developments on either side. But it does not look as though he can show this about conception. Whichever ‘moment’ is settled upon as the salient one will be arbitrary in the sense that no reason can be adduced for distinguishing it from the next closest, especially since the moments can always be individuated evermore finely. Moreover, it would be unfair to reply that the moments constituting the process of conception are too closely gathered to give rise to a problem of arbitrariness, but that those constituting birth are not. The problem of being unable to non-arbitrarily distinguish one moment from the closest neighbouring ones, if it is a problem, is exactly the same regardless of how long or short the time slices are. On the conception theorist’s reasoning, the logical ‘slippery slope’ moving the threshold of personhood later and later in human development would still exist even if the increments of development are minute.

Further to this, it will seem that whatever considerations supporters of the conception threshold can bring to defend it against the sorites problem will also be available to those who support post-conception thresholds such as birth. It might be argued, for instance, that sorites-susceptibility at the borderline of personhood does not obviate the existence of clear cases, and that humans are clearly persons once conception has certainly been completed. But those defending the birth threshold can respond thus to the sorites problem as well. They can contend that the progressive nature of birth does not preclude there being clear cases on one side and the other: humans that are clearly born, and those that are clearly not. If the suggestion is that sorites-susceptibility rules out any proposed threshold of personhood, some explanation must be given as to why it does not rule out all of them, conception included.

  • [1] The descriptions are taken from Michael Tooley’s contribution in Philip Devine and others,Abortion: Three Perspectives (Oxford University Press 2009) 44—5 and Keith L Moore and TVNPersaud, Before We Were Born: Essentials of Embryology and Birth Defects (5th edn, WB Saunders 1998)36—7. Tooley credits many of his descriptions to Neil A Campbell, Biology (4th edn, The Benjamin/Cummings Publishing Company 1996).
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