On Patheos, pro-choice blogger Bob Seidensticker has formulated an argument for abortion that he calls the "Spectrum Argument" for abortion.
First, I'd just like to take a moment to say that it's refreshing to find a pro-choice blogger who actually attempts to make a logical argument for his position, rather than the rhetoric, demonizing of pro-life people, and just all around fallacious and otherwise bad arguments which are the normal fare on Patheos, as well as other pro-choice sites like RH Reality Check and Jezebel. So kudos for that.
Seidensticker's argument is that there is a spectrum of human personhood during gestation. He asks us to consider a spectrum from blue to green, where there is a continuous change. It's difficult to know where blue ends and green begins, but we can all agree that blue is not green. Similarly, it's difficult to know where a child ends and an adult begins, but we all know that a child is different from an adult.
So his argument seems to be something like the following:
1) Human personhood exists as a spectrum, from "not a person" to "full-fledged person."
2) Human fetuses fall toward the "not a person" side of the spectrum.
3) Therefore, human fetuses are not persons.
He gives a supporting argument. He argues that a brain with 100 billion neurons doesn't think 10^-11 times as fast as a brain with a single neuron. In fact, he asserts, it doesn't think at all. By the same token, the differences between a newborn and an adult is trivial compared to the difference between a single-cell zygote and a one trillion cell newborn.
The immediate problem with this argument is that he gives no attempt to argue at what point we actually do become persons. This is a problem, because if there is the slightest chance that we may be killing human persons, then we ought not to kill them or we risk doing serious moral wrong. The argument "we don't know when we become persons, we just know the unborn don't qualify" is simply an ad hoc justification for abortion.
He resorts to the tired old arguments that an acorn is not an oak tree (no, but it is an immature oak tree), a silkworm is not a dress (of course not, because dresses don't develop from silkworms; it is made from the silk the worm produces), etc. This kind of argument just denotes a complete lack of knowledge of basic biology. I have dealt with these arguments elsewhere.
The spectrum argument fails to adequately address the fact that there is a continuity of human development that begins at fertilization and doesn't stop until after birth. Logically, that suggests that teenagers are "more of a person" than toddlers, because they are more developed, so teenagers should have the right to kill toddlers for any reason they want. And since adults are more developed still, they should have the right to kill teenagers. Then, of course, you start to lose certain developmental abilities as you get older, such as memory, motor functions, etc. So the elderly are less valuable and should be able to be killed by an adult for any reasons whatsoever. This is clearly an absurd argument, but it is the logical conclusion of the spectrum argument.
Not only is his argument absurd, but his supporting arguments don't do the work he needs them to do in order to support his main argument. It may be true that a brain with one neuron doesn't think nearly as fast as a brain with 100 billion neurons, but he misses the point that it is still a brain. It is just an immature brain. It hasn't had the chance to develop into a fully mature brain. Similarly, the unborn may be less developed at the single-cell stage than the 100 trillion cell stage, but it is still a human person at that stage. She is just an immature human person at that stage. She looks and acts exactly as a human being does, as every human being does, at that stage.
Seidensticker's assertion that he's an expert at babies doesn't add anything to his argument. I have had a hand in raising three of my nieces. Pro-life people have babies, too. The dictionary lists one definition of "baby" as "a human fetus." In fact, the term "fetus" in Latin means "offspring." It's true that pro-life people can use the term to add emotion to their argument, but it's equally true that pro-choice people often insist on using "fetus" or "embryo" in an attempt to dehumanize the unborn to make it more palatable to justify abortion.
So his comparison of the pro-life argument to PETA's slogan of "a rat is a pig is a dog is a boy" is simply a false analogy. The unborn are full human persons, a claim I have amply justified before. And it's not hard to see how his argument fails. Joseph Merrick, the Elephant Man, did not appear human, and yet he clearly was. Being human in appearance does not make one a human person. What makes one a human person is their inherent nature as rational agents, a nature that is shared with the unborn. The reason that you have the present ability to think rationally and to even read this article is because the zygote that you were at the beginning of your life had the inherent capacity to develop it. As Christopher Kaczor notes, hedgehogs don't develop rationality because it's not in a hedgehog's nature (Sonic notwithstanding).
Seidensticker's point about how evangelicals thirty years ago supported abortion is simply irrelevant. Many people, from evangelicals to atheists, have become pro-life over the last thirty years. That's a testament to the strength of the pro-life position! Seidensticker's history is just a red herring, as it detracts from the actual argument.
Seidensticker finishes off by arguing that the pro-life argument fails because it fails to account for the spectrum. Seidensticker did a good job of representing the argument, but a poor job of refuting it. He didn't refute any of the premises, he just asserted (with weak evidence) that human personhood is a spectrum. Let's take a look at the argument as he presented it:
(1) Human life begins at conception;
(2) It is murder to take a human life;
(3) Abortion is murder and should be considered immoral.
This argument cannot be done away with by asserting a spectrum. The "human life" in this argument is clearly biological human life (and he even admits that from conception the zygote is a human biologically). If it is wrong to take a human life, then Seidensticker has affirmed the argument, not denied it. His "spectrum" argument is clearly an argument against personhood, not biological human life. So the argument as presented here succeeds. And as the icing on the cake, I have also soundly refuted the "spectrum argument."
Human development may exist on a spectrum, but human value clearly does not. The only clear place to draw the personhood line is at fertilization, because that's the clear place that someone begins. The sperm and the egg cease to exist and the new human being comes into existence. Then they are on a lifelong path of development that begins at fertilization and doesn't end until they're an adult. If abortion is going to be justified, clearly it won't be justified in this manner.