Personhood has to have some meaning and some purpose.
I would agree, but I think what you mean to say is that pragmatically we need to draw lines about how the laws apply. That is not what I would call a 'purpose' of personhood. Or rather, it's not what I would call personhood at all. A legal fiction - even a necessary one - has nothing to do with the actual facts of reality. For the record I'm not talking about the current state of law, but rather about the actual intrinsic worth of a human being, a matter which the law has no business commenting on.
The cutoff for death is lacking brain function. Ergo, it would be bizarre to define anything that doesn't have brain function as a person. This is a completely rational cutoff. Prior to brain function, an embryo isn't much different from cancer. The rather absurd criteria of 'unique human life' is equally well met by dead people, cancer, a random cheek swab, or cells pooped, urinated, or otherwise shed.
I was not offering a rigorous definition of personhood. In fact I said outright that it would be unbelievably complex to even formulate such a definition. What I mentioned was an example of why common sense would suggest that a new embryo is not the same as cancer, poor, or a cheek swab. If you think your examples are "common sense" versions of the same as an embryo in terms of being a unique being, I think I would agree with JoshuaD that you seem to be deconstructing language to the point where there are no definitions, only legal fictions.
There is zero rational basis for extending personhood to an embryo
Maybe so...and if one day I see a demonstration or a rational argument showing why your belief is right, I'll take it seriously as a proposition. But saying there is zero basis is quite the claim. Do you mean to say that you have at your disposal the full powers of physics and biology, have solved the cosmic moral questions, and know all there is to know, such that you can eliminate any possibility? I know this is grandiose sarcasm, but I'm not sure how else you can make the statement you just made and know you are right. Otherwise I think you would have to stick to - at best - that you so far haven't seen anything convincing to you. And that's fair enough if that's what you meant. But then I would assume that you ought to follow it up with "and I'll keep my eyes open for information that might change my mind."
Is personhood distinct from "human life"? Is there such a thing as human life that does not contain personhood?
Do you consider your poop to consist of multiple people? Your shed skin? Your urine? A cheek swap? Is cancer a person? If not - they no 'human life' is not the same as 'personhood'. Also legal personhood ends at brain death, even if all of the other organs are perfectly functional.
This is more of a sophistic argument than I am used to hearing from you. Maybe this topic makes people edgy or something. The best I can figure is that you're equating an embryo to all of these things genetically. Aside from the fact that I find it hard to believe you really think they are analogous, do any of these things grow into people if you simply do nothing? If so we have bigger problems than abortion! Regarding legal personhood, it's not what I'm discussing so I'll take a pass on that point.
We already have a legal definition of personhood that is universally accepted - a functioning brain. No functioning brain and you are legally dead.
That's nice, but I hope you're not going to hang your moral hat on what the current state of law says. Not that you meant this, but fascist governments intent on ethnic cleansing tend to declare the non-personhood of the undesirables in their borders. That they can make it law does not make it right. Within democratic governments it is likewise possible to establish rule of law that includes immoral or simply incorrect elements. Now if all you're talking about is that there's currently no confusion in hospitals about who should or shouldn't be allowed to have the plug pulled (and we'll ignore euthanasia for now) then of course, there must be a regulation of some kind. But when the popular - or legal - definition is being challenged on moral grounds, you can obviously see that it's a circular argument to point to the law and say it's already an accepted fact.
Well if you don't have a brain - you can't experience or do anything and there isn't anything to differentiate you from cancer, or random cells in a petri dish.
"Don't have" is an awfully prejudicial term used in this context, when the thing to be had will inevitably exist simply with the passage of time. It's as absurd as saying that an embryo is "uneducated riffraff" on account that it's never gone to school. Well not yet! Give it the chance to fail before saying it's an educational nightmare for the poor tyke. There are many, many things you haven't gotten yet because the time for them is ahead, and yet you would define the current state of that being by what it lacks as of yet? Now if you showed me an embryo with a genetic defect wherein we knew for certain it
would never develop a functioning brain (or one at all), we could have a discussion about whether that could ever be a person. And that's assuming I accepted it's all about the brain. But even if it is all about the brain, the fact that one will grow in a few months seems to me...and I think to pretty much most people...to make the embryo different from a cheek swab. I think what you are missing here is flow and process. The process an embryo is going through, and the flow - by which I mean the gradual differences over time - are the most relevant issues about its identity. Just take a simple chemical process like the electrolysis of water, and imagine you turn on the current, but an infinitesimal amount of time after the current enters the water someone asks you what's in the vial. If you answer "nothing but some water" you would really be misstating what is going on in the vial. There is water there, but also current that, unless an extraordinary force intervenes, will begin to break the H2O down into gases. While I agree you wouldn't answer "gases are in the vial" at the point where the electricity hasn't finished its job yet, it would also be inaccurate to just say it's water and that's it. What it is is a substance going through a process, where you know where that process will go and what powers it.
Similarly if you transferred every organ of a body but replaced the brain with a computer chip that could substitute for the brainstem (i an body part incubator) - it would again universally not be considered a person.
And interesting sci-fi premise, but let's leave off it for now. I don't think discovering what the results of this experiment would be is trivial.
You may feel that there should be protections for clumps of cells with 'potential' to become persons, but that isn't personhood.
Depends what you mean by "potential". If you mean in teleological terms that it's what those cells are for (i.e. to become persons) then we are still left with the question of whether those cells are persons. You are begging the question by assuming your conclusion as a premise. And if you mean something more physical, like as in potential energy - like if you raise an object in the air and drop it, it has the "potential" to fall - then this is more of an inevitability unless you intervene. But maybe you mean something more like "it could happen, and maybe not, who knows." I sort of feel like you're using the word potential in this sense to imply uncertainty, but if so that seems to me the weakest version of what it could mean in this context, because likelihood of success is not really relevant to whether the cells are in fact a being in process of growing to completion. Maybe you meant something else by 'potential' here, but it's not clear what it could be that solves why suddenly it can't be a person.
The heartbeat isn't a rational basis. We don't think people are dead if we have transplanted a heart pump that doesn't function by beating.
Well some people do seem to think it's a rational basis, but I'm happy we agree on this point.
If you think that an embryo should be a person - you have to articulate why a brain dead body isn't a person, why cells in poop, cancer, a petri dish, urine, etc. aren't also people.
I'll only address brain death, as I've covered the others already. Now for what it's worth pro-life people tend to (I think) consider brain dead people to still be people, so whether or not you agree with that it's a consistent position. If you are asking me whether it's reasonable to consider a brain dead person on life support as a person, I would have to say that I find myself lacking enough information to make a declaration about that. In other words, if a person's brain dies, should that be considered as them having died - I don't know. But I'm not sure what that has to do with a person whose brain has not died - by virtue of it not having been constructed yet. Death would seem to be the operative issue here: if a person dies their remains are no longer a person. And that's really all we can say that seems clear on a common sense level. Once you start getting into whether an embryo is similarly not alive since it's - pre-dead?? - not developed yet, I don't know what to say. It sounds like a really weird argument to say that if a person hasn't developed into a life form yet then they are like a dead person. I don't think that's what "dead" means, and your argument rests on comparing an embryo to a corpse (a morbid comparison, incidentally). And in any case it's still circular as it was above since in order for even this weird argument to work you have to assume that an embryo is not alive (which gets us back into the quagmire of defining life).
It is frankly the most common sense answer I can think of, if what we are asking is when the genetic material becomes unique, neither of the mother nor father.
This implys that twins and clones shouldn't have rights. Any definition that is about uniqueness of genetic material is inherently flawed.
Does it? I think it's pretty clear that
the twins are unique from the parents. That they are genetically the same as each other is a fascinating feature in nature, but genetically identical twins are not in fact the same as each other, they are still different. Unless you are arguing that twins are in fact the same person?
Regarding clones I would have to reply that we're talking about the results of sexual reproduction and how to interpret that. The moral and ontological issues regarding clones are really a new area, and frankly my advice would be that if it's already a fiasco discussing the kind of reproduction we already know, it will be worse when discussing clones. Bringing the more difficult case into play when examining the standard case is probably not going to be that helpful. Long-term I expect that a new understanding of when life begins might have to be introduced when discussing cloning in the future. Even pro-life people might agree that there can be variable timings of when life begins, but that for the case of sexual reproduction it's at conception.
As a side point I have read some sci-fi, and already recall some discussions involving Pete (I think) about how life may be seen as a continuous process (or even being) that does not in fact have cutoff points; so that the answer to "when does a single human life begin" would be answered in a way like "a few hundred thousand years ago." I kind of like that answer, not that it helps us establishing laws.
Also I think a dog, cat, dolphin, elephant, or ape has far more claim on personhood than a fetus does. They actually have personalities, can experience emotions (love, hate, fear, anger, envy, joy), learn, and socialize.
This sounds like an aesthetic argument, based on maybe level of sophistication, or maybe the amount to which you recognize features that you possess. I don't believe this is a rational argument; or at least it can't be rational unless it's based on something other than comparing it to animals. We're talking about an issue that can't be other than rooted in first principles.
If you declare embroys persons, then you have a moral obligation to try and save all of the embryos flushed down the toilet
Yes.
and you have to give a rational basis for why we aren't morally obligated to rehabilitate cancer and various shed cells into embryos. It is a fairly trivial process to convert somatic cells to embryos, so you pretty quickly get into a 'all cells are sacred'.
I really can't understand why you think this follows from the first point to which I said "Yes."
Legally we require and endpoint of personhood - and brain death is fairly universally accepted - so morally and logically 'brain life' seems like a strongly compelling beginning point for personhood.
If I take your "so" very seriously, then you are arguing that moral and logical truths originate in or are derived from laws. But I don't really think you believe that.
Medically a fetus is incapable of life without the womb until it has a functioning brain.
Medically you are incapable of life without the womb we call Planet Earth, with or without a brain. So what?
Once we're talking about independence I don't see how you can quantify that.
Sure you can - can the life take in nutrition and eliminate waste without physical integration with another being (or technology that mimics that physical integration)? Or more simply is the life dependent on another being forming a special organ specifically for the survival of that life? If not it is incapable of any sort of independence. Requiring a chorion, placenta, and umbilical cord for nutrition and waste elimination is a pretty strong argument that a fetus isn't independent.
Forgive me, but are you actually arguing that an embryo cannot be a person as a result of the fact that it's an embryo? The facts that literally define it as an embryo are your definition here of what wouldn't constitute a person. Well I mean, sure, you can just as well say it isn't because it isn't. That's consistent, but it isn't rational. And yes, I know you're also including people on life support within this definition, but it's not the same and you know it. An embryo is not a dying person on life support by any rational definition. As an aside, your reply was about the fact that it can't be quantified, and you disagreed but did not try to quantify it...
There are weaker versions of dependence (such as dependence on another being for immune cells and antibodies).
There is also a prevalent thought in modern society that people are really not too dependent on others for survival, that they 'take care of themselves.' It's completely absurd from every standpoint (economic, medical, ecological, psychological, etc). Hanging your rights on how 'independent' you are isn't something I would recommend...
You don't like the answer that comes from reason, so declare it impossible to use reason to think about the answer. It isn't that reason doesn't provide strong arguments for particular definitions of personhood, it is that you would rather reject reason than come to a conclusion that is contrary to your preference.
I don't like bad arguments. Based on my comments above I named one coherent argument (the current pro-life one) but left the matter open whether there were others. I have not been trying to make a case for the pro-life position here. Rather, I am making a case against the particular pro-choice positions presented here. This isn't a labyrinthine trap designed to lead directly to pro-life. I would legitimately be interested to see good arguments about other visions of what personhood might mean, or where it begins or comes from. I am talking philosophy, not religion.