I have been following this thread for a while, and I would like to add my perspective on the debate.
I think that the fundamental question in the abortion debate is the personhood of the foetus. If the foetus is a person, then abortion is the act of killing another human without cause, and is therefore wrong. If the foetus is not a person then abortion is merely a medical procedure which has little more ethical implications than something like an appendectomy, and should not be prohibited.
I think under any reasonable definition of the words it is clear that a foetus is both “human” and “alive”. The question is to whether this human and alive status implies a personhood and therefore whether rights such as the right to life should be granted to the foetus. There are two main ways to assign personhood. The first is to assign them ontologically, that is to say that all things that are human necessarily have personhood. Followed through since the foetus is human from the point of conception, then the foetus has personhood since the point of conception. This is essentially the pro-life position.
The second way of defining personhood is to do it functionally. This method implies that personhood is connected to some property or ability of the being in question, rather than the nature of the being. Using a functional definition personhood can be granted to a foetus (or a child, as the case may be) once they gain the functional characteristics that defines personhood. So for instance “viability” (of which I will say more later) is often presented by the pro-choice side as a functional characteristic that is required for personhood.
Of the two approaches I think that the ontological approach is by far the more logical. The main problem with the functional approach is that there is no clear functional property that can be used to define personhood. The wide range of possible points demonstrates this. Further of those that are put forward there is often nothing unique about them.
So for instance take intellectual ability. Commonly it is seen that all animals have no personhood, and therefore it is fine to kill animals for food, clothing, whatever. However if we wish to use brain function to define personhood then there are many animals that exhibit much higher brain functioning that a third trimester or even newborn baby. If brain functioning is our standard, and we give personhood to newborn babies, then personhood should also be granted to a wide range of animals which exhibit similar brain function. There is nothing unique about the functional ability of a newborn baby in this regard. This is essentially the argument of Peter Singer for animal liberation, that if we are going to define personhood functionally, then we should be consistent.
Personally I think that a lot of the typical pro-choice distinctions of when personhood begin have much more to do with allowing abortion, rather then a sense of intellectual rigour. So for instance take “viability”, which has been bandied about quite a bit (and as I understand it is also the line drawn in Roe vs Wade). As a definition of personhood it has a number of flaws
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First and foremost “viability” is not a property of the foetus itself, but is a property of the society around it. What is “viable” in some places and times is not viable in others. 500 years ago viability was somewhere around 36 weeks. With increases in technology we have gotten it to somewhere around 24 weeks. In another 500 years we may have invented mechanical wombs so that a foetus is viable from the point of conception. It is absurd to try and define something fundamental to a person such as personhood through something that is not a property of the person themselves.
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To illustrate this there are several absurd situations that may arise. Say for instance the point of viability in the US is 24 weeks, but because of lower levels of medical care the point of viability in Somalia is 30 weeks. So a woman in the US, when she reaches 24 weeks as a “person” inside of her, and cannot ethically get an abortion. What happens then if at 27 weeks she travels to Somalia? Does the foetus stop being a person now that they are effectively unviable? Should the mother be charged with murder since she has destroyed the personhood of the foetus? And can she now get an abortion since the foetus in no longer viable?
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“Viability” as a concept is almost meaningless anyway. Babies born premature often require intensive medical care to live. Even babies born full-term require constant care (feeding, nurturing, ect) or they will die. Given this “viability” doesn’t mean “can survive on their own”, viability simply means “able to survive without a specific means of support”. Given the level of care that premature babies need there is no logical reason to exclude the support of a human womb from the list of possible care. Why is a baby that is able to live only in a womb or a humidicrib and more or less a person than a foetus who is only able to live in a womb? In both cases without serious levels of care the child will die.
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The objection that until a foetus is “viable” then the care of the foetus comes down to one person, the mother, is commonly raised. The general principle advanced here is that since only one person is capable of caring for the foetus, then it cannot be a person. This is clearly absurd since this seems to be the only situation that this applies in. There are many situations that we place ourselves in where our life is dependant on the actions of an individual. Such situations in no way negate our personhood, and therefore neither should it in this case.
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If “viable” is the distinction for personhood, then again there are many animals that are not given personhood that clearly exhibit more viability than even a newborn infant. So for instance while newborns require constant care cockroaches live and breed not just in human indifference, but in human opposition. Clearly their life is more “viable” than a newborns, yet they are not considered to have personhood.
Since then there is no reasonable logically consistent definition of personhood besides that of conception, I would say that personhood begins at this point and therefore abortion is wrong.