At the end of an otherwise quite interesting Slate piece, which discusses the potential development of a "consciometer" — a medical device for measuring consciousness — the author shifts from science to law and morality (paragraph break added):

As leading neuroscientist Michael Gazzaniga . . . describes . . ., current neurology suggests that a fetus doesn't possess enough neural structure to harbor consciousness until about 26 weeks, when it first seems to react to pain. Before that, the fetal neural structure is about as sophisticated as that of a sea slug and its EEG as flat and unorganized as that of someone brain-dead.

The consciometer may not put the abortion issue to rest — given the deeply held religious and moral views on all sides, it's hard to imagine that anything could. But by adding a definitive neurophysiological marker to the historical and secular precedents allowing abortion in the first two-thirds of pregnancy, it may greatly buttress the status quo or even slightly push back the 23-week boundary.

There is another possibility. The implications of the consciometer could create a backlash that displaces science as the legal arbiter of when life ends and begins. Such a shift — a rejection of science not because it is vague but because it is exact — would be a strange development, running counter to the American legal tradition. Should a fundamentalist view of life trump rationalist legal philosophy? Roe v. Wade considered this question explicitly and answered no. For nonfundamentalists, that probably still seems right.

This is a deep error; and it can be called "scientific fundamentalism" because of its tendency (similar to that in the most unpersuasive versions of religious fundamentalism) to assume that If It Isn't In [Science / The Bible / The Koran], It Doesn't Matter.

What rule we should use for deciding when someone should have the legal right not to be killed is not a scientific question. Applying the rule may be a scientific question; if we decide that only entities that have consciousness have the right not to be killed, then science can tell us whether John Smith has consciousness. But deciding on the rule is simply not a scientific issue: It's a matter of moral judgment, which science isn't equipped to provide. Science can't tell us whether the legal right not to be killed vests at conception, at viability, at consciousness, or at birth; nor can it tell us when the right dissipates.

Let's take a simple hypothetical, which I hope can persuade even people who feel a deep intuition that the right to be killed is closely connected to consciousness. Say it turns out that there's a disease that temporarily caused someone to lose consciousness — not just in the sense of sleeping or getting knocked out, but in the sense of mental functioning largely ceasing — but there was every reason to think that in several months the person would regain consciousness. Would it be OK to kill him then? (I realize that this is likely a counterfactual hypothetical, but I think it's still useful; and one can certainly imagine some future medical procedure that would turn off someone's mental functioning but keep the potential for functioning present, by stopping the brain from atrophying.)

I take it that the answer is "no," because the test wouldn't simply be whether the person is conscious; potential for consciousness, perhaps coupled with some other factors, would suffice. But why not then for a week-old fetus, which also has the potential for consciousness?

Naturally, there are answers to this; the hypothetical isn't meant to support the life-begins-at-conception position. One could, for instance, argue that the test should be whether the entity either has consciousness, or has had consciousness and seems likely to recover it. Or one could say that the test should be whether the entity has the bulk of the physical equipment needed to support consciousness, even if consciousness is temporarily absent. These may or may not be perfectly sensible arguments. But science can't prove the validity or invalidity of these arguments. Nothing in biology, chemistry, physics, or any other science speaks to whether these tests ought to be the tests for a right not to be killed.

So it makes little sense to say to someone who believes that the right not to be killed begins at conception: "You are a fundamentalist who wants to displace science as the legal arbiter of when life ends and begins, rather than the rationalist legal philosopher you ought to be — see this conscionometer that clearly proves that this fetus doesn't have consciousness, has never had consciousness, and currently lacks the physical equipment needed for consciousness, even though in several months it is nearly certain to have consciousness." The conscionometer answers a particular question, but it tells us nothing about why this should be the right question. And if you've concluded that "does it have consciousness?" (or some variant of that) is the right question, that's a moral conclusion, not a scientific one.

I can certainly see why some people, especially those who love science, might want to believe that science should be "the legal arbiter of when life ends and begins," and why they want to think that their moral intuitions are simply "rationalist legal philosophy" and any contrary moral intuitions are "a rejection of science." But they are mistaken; and they are in their own way victims of a fundamentalist (and fundamentally erroneous) perception of the role of science.

UPDATE: I had a problem enabling comments, so I've posted a new post (linked to below) that lets you post comments.

Please post them here; please keep them on-topic, substantive, and polite. (Because of a glitch, I couldn't add them to the original post.)