In any case, I finally got around to looking at the recent statement put out by a number of philosophers on the question, and it is a good example of how completely useless philosophers can be on these kinds of issues. The statement says:
What has not been discussed much is whether a 12-week old foetus is a person entitled to constitutional protection. What makes this particularly problematic is that the issue hinges on a complex philosophical question that has no straightforward answer, namely ‘What is a person and when does a person begin?’
If we accept that personhood is indeed "a complex philosophical question that has no straightforward answer", why would we also think it is even relevant to the question of whether one should retain a constitutional amendment that doesn't even use the word 'person'? The Amendment in question reads, "The State acknowledges the right to life of the unborn and, with due regard to the equal right to life of the mother, guarantees in its laws to respect, and, as far as practicable, by its laws to defend and vindicate that right." I don't see anything explicitly about personhood here. And why anyone should spend one's time on questions that have "no straightforward answer" when discussing constitutional issues is beyond me.
The obvious reason it's brought up (besides its being something the philosophers in question think they can sound clever discussing) is that it is being assumed that that what is not a person is not entitled to constitutional protection. Put precisely that way, it's obviously false -- every constitution provides protections to things that are not persons -- but presumably, by 'entitled to constitutional protection' what they really mean is 'having a right worth acknowledging by a constitution' -- in this case, a right to life. Taken so, the key issue would be what a right to life is (which would tell us, without bare assumption, what kind of thing it could apply to), not what a person is, but in an era in which people discuss whether rivers and ecosystems and chimpanzees and species can be said to have rights, they've decided to stake their whole and entire claim on personhood, as if they all just woke up from having been frozen in the eighties.
In so doing they (unsurprisingly, perhaps) make a further common mistake of philosophers discussing matters like this, namely, they assume that if personhood is relevant it must be a metaphysical rather than a forensic and practical notion of personhood. Here is an argument that gets used, in different variations, in considering animal rights: "Persons have rights; but the assumption that animals with such-and-such characteristics are not persons is a matter that can be controverted; supposing even that we are at an impasse and there is 'no straightforward answer', the law should, to the extent practically possible, play it safe in matters of rights and assume that rights are more rather than less widely spread, particularly given the atrocities in which law can be complicit if it decides to start dictating what is not a person." This is obviously itself going to be a controversial argument in many cases, but as arguments go, it is perfectly sensible, and could be easily adapted to this situation. And while it depends on the notion of personhood, it does not depend on any metaphysics about personhood; what it depends on is a notion of law and its purposes, and is arguing that, given that notion of law and its purposes, we should legally count animals of such-and-such characteristics as persons for those legal purposes.
Or take another line of argument. "There is practical reason to have laws against fetal homicide, namely, that if one doesn't then, even aside from assumptions of fundamental rights, it becomes very difficult to do justice to parents. The easiest legal way to make laws against fetal homicide work, however, is to treat the fetus legally as a person existing under particular conditions." No doubt there would be people who would disagree with such an argument, but it's a perfectly appropriate argument for legal purposes. Law is a practical field. One does not need the finer points of metaphysics to do it. And here we have an argument for attributing personhood that does not require any metaphysics at all; it depends not on the metaphysics of personhood but on the practical question of what the easiest way to do justice to a third party would be. The concept of 'legal person' or 'juridical person' is not a secret; it's widely known. So why would one assume that the metaphysical notion of personhood would be a necessary, and not just a sufficient condition, for treating something as having rights, even if one connected rights directly to personhood in the traditional way?
It gets worse. They go on to say:
Influential figures like St. Augustine and St. Thomas Aquinas held that a foetus is not a person until it begins to move, which they took to be 40–80 days after conception.
This is entirely wrong. Neither Augustine nor Aquinas are talking about personhood when they discuss these matters; they are talking about the standard view of what is involved in conception of a human being. The standard biological view in their day regarded human conception as an extended process over time; it was literally a sort of cooking process as the materials developed in the heat of the womb. Quickening, i.e., the point at which the fetus begins to move, plays no role at all in the thought of Aquinas. The only time Aquinas mentions the 40 to 90 day point he is talking about Aristotle's view, not his own (he never, as far as I am aware, commits definitely to any particular timetable at all). Quickening did not begin to have much of a presence in discussions of conception at all until the early modern period, and then it was a purely legal device for deciding how serious the penalties should be in various legal cases (e.g., if someone harmed a pregnant woman and caused her to miscarry). And, again, prior to the egg-and-sperm model, the dominant view of conception was that it was a process lasting several weeks; it shows nothing but historical ignorance to frame the discussion during this period as personhood arriving weeks "after conception". Augustine treats as the key characteristic not movement but sensation. This level of non-research in people trying to pull out their credentials in order to influence a political situation is embarrassing. What good is your status as a "professional philosopher" if you are going around making statements about texts that you've never bothered to read?
It gets even worse:
We grant that the question ‘when does a person begin’ is complex. But because the constitution is the backbone for all law in the state, it should be confined to highly plausible restrictions on the law that more or less everyone can agree with.
This is not how constitutions work. This is not how constitutions have ever worked. There is no serious theory of constitutions that would take this as a reasonable principle. Did not a single philosopher signing this document pause enough to ask, "And what would be the result if this were applied in contexts involving slavery, or mistreatment of minorities?" Is every one of the signatories so ignorant of history that they completely missed the fact that constitutions are built not on "highly plausible restrictions" that "more or less everyone can agree with" but (at most) on compromises that give concessions to as many major groups as possible, because you can't practically build a constitution entirely out of prior agreements? Do they honestly think that most constitutions in the free world are built entirely out of things that are "highly plausible" and that "more or less everyone" originally agreed with? That the purpose of a constitution is just a formality to summarize what practically everyone accepts anyway? Constitutions cannot do one of the things they are morally supposed to do -- protect the rights of the vulnerable -- on the principle given; constitutions are not in fact ever written on the basis of the principle given; and the principle quite obviously shows up here only in order to get the conclusion that they want.
Schopenhauer, I think, says somewhere that arguments are not like cabs; you cannot ride them to your preferred destination and then get off. Apparently none of these philosophers have learned that lesson yet; or else (more likely), they just decided to sign without any regard for the rationality of the argument, because they already agreed with the conclusion. The whole thing is just awful sophistry.