And when I suggest that the current framework of abortion legality is based on unscientific (basically religions) claims, this is the sort of thing I'm referring to. "Well, we've defined this abstract concept of personhood detached from actual humanity that we will only ever bring up for fetuses and dolphins, and that's why abortion should be legal." You really are taking a pseudo-religious stance here, making a metaphysical claim that you personally believe to be self-evident.
If I were to make my counter claim against you, something I feel simply to be self-evident, any definition of "personhood" which is detached from the actual humanity of the person is a shallow excuse for bigotry or murder.
Your argument here is
scientism: the mistaken attempt to explain with science what is outside the remit of science.
This is philosophy: ethics. As David Hume noted via the is-ought problem, you cannot objectively prove morality, and all those who have tried (like pseudophilosopher Ayn Rand) have failed miserably. Science provides knowledge that can inform an ethical position, but it does not prove ethics.
The definition of personhood is philosophical and legal, a social construction. Science cannot "prove" what a person is: there is no measurable matter or energy to constitute personhood that natural sciences can recognise it as an independent phenomenon. It is what we say it is. But nor is it entirely arbitrary, because it fits in a wider scheme of our understanding of people, things, and their interrelationships - some of which science does contribute to. The notion that something is a human because it has a soul, for instance, is something that must fall flat on its face in terms of science.
The dominant definition of "personhood" relates to the idea of a being capable of independent, intentional action - often with a notion of
moral action. If you want to define a human as an object made of human cells with human DNA, science can prove whether something is or not. But if your hand is chopped off, why is that not now an independent human being? It is after all made of of living human cells with human DNA, which are capable of continuing to reproduce and working with each other to create functioning tissues. Should we not call every single petri dish of HeLa cells worldwide persons, and give them the right to vote? Say intelligent aliens came to earth and said "Hello, we come as friends", should we just kill them with impunity because they aren't human therefore cannot qualify as having any recognisable rights?
So science can step in here to provide answers... does that thing have the capacity for action that makes what we take to be a person? If no, it's not a person, and it does not require the rights of a person. As already pointed out, it is that degree of capability for independent action which I was getting at when I mentioned sapience, earlier. A dog thinks and feels and has volition, although below that of a human. An ant thinks and feels in a way, but quite likely does not have anything we could meaningfully call volition, more being a simple, programmed bunch of cells. And thus it is we constructed an entire hierarchy of rights, all of which effectively derive from a consistent concept.
And so we place fetuses in that hierarchy, and (early enough) they do not really warrant protection any more than a petri dish of HeLa cells does. They certainly do not merit priority over the mother, who most certainly is a person capable of independent action - and with rights of bodily integrity as also agreed in our great, big, societal ethical framework. You can smash a great, big hole in that consistent ethical framework if you really want - such is the way of socially constructed things, but we have words for egregious double standards and exceptions, such as: "injustice".
