One argument commonly put forward for why the embryo is not a human in the first 14 days or so of life, before implantation, is that it may yet divide into twins. If it can become two individuals, the logic goes, how can we say it is now a person? It must be in a pre-person state until it becomes two separate people.
Twinning is basically a cloning process, or a form of asexual reproduction. The one embryo splits into two pieces, which each grow into identical twins.
If we consider this from a purely atheistic perspective, the argument rapidly falls apart. Just because something may later become two individuals doesn’t mean it is not an individual before then.
If a bacterium divides to produce two identical clones, does that mean it was not an individual bacterium before the cloning? Of course not.
If the technology ever becomes available to clone a human adult (some claim it has already been done), a human adult could later become two humans. Does this mean the adult was not a human before the cloning?
The embryo is one human. If at some later point it can become two humans, that does not detract at all from the fact that it is already human. It only means that by killing it you have cut off the potential for two lives rather than only one – which is twice as bad.
Now if we add the Christian perspective, that humans are not only physical but also have a soul, you may be able to say the embryo is not human before it divided because the individual twins were not given souls until they separated.
But how do you know? This is pure speculation. God may do that, or He may give one soul at conception and a second one when twinning occurs, or (knowing twinning will occur) he may give both souls from the start… The Bible never says. We can speculate for ever but will never reach an answer. It is best to forget this line of reasoning and focus on the physical.
So whether you are atheist or Christian, the twinning argument holds no water. And anyway, most embryos don’t twin – how many identical twins do you know?
For a far more detailed refutation of the twinning argument, check out this excellent article by Alexander Pruss of Baylor University (hat tip MandM). Matt’s response is here, but he does not appear to really counter what Pruss says, just disputes some minor points.
I have discussed other issues around abortion in these posts.
Samuel Dennis 
March 23, 2009 at 10:34 am
I would not describe Matt’s response to Pruss as a refutation. Matt does not essentially disagree with what Pruss wrote at all.
The issue Matt is talking about, the one I didn’t really get that he thinks is persuasive, Matt says that Pruss did not really address.
Whilst Marquis’ article does not appear to be available on the net, Matt can access if via the philosophers data base – I will get him to get you a copy of it.
March 23, 2009 at 10:35 am
My own position is pretty much yours but knowing Matt I suspect my own position is too simplistic as there is no way Matt would get say an issue was important without him having good reasons for doing so – hence why I intend to read Marquis’ argument myself.
March 23, 2009 at 10:53 am
You’re right Madeleine, I have changed the wording to not call his response a refutation. It would be great if he could get me a copy, but no worries if he can’t.
“…knowing Matt I suspect my own position is too simplistic as there is no way Matt would get say an issue was important without him having good reasons for doing so…”
You’re a very supportive wife even when you disagree. Good on you!
March 23, 2009 at 11:59 am
“One argument commonly put forward for why the embryo is not a human in the first 14 days or so of life…….”
Your are making this stuff up right – in all my not yet 65 years and countless discussions and arguments I have never hard or seen the argument that a embryo is not human because of the existence of twins.
It may be an argument but it sure is not commonly held!
You exaggerate for effect dear sir!
Sb
March 23, 2009 at 12:30 pm
Sb, it may just be an argument commonly used among those who debate the morality of abortion philosophically. I’ve heard it a lot, but maybe I just discuss the issue more than you do.
March 23, 2009 at 12:42 pm
As I said – I had never heard it before, the idea of arguing that a embryo is not human is amazing given that it is composed of Human DNA.
if it is not human what is it?
Sb
March 23, 2009 at 1:21 pm
Sb
The argument is actually fairly standard in the literature. Paul Ramsey raised it. Norman Ford has raised it. Don Marquis discusses it in his writings on abortion. Frank Beckwith discusses it in his writings. Peter Singer advances it. Baruch Brody in his work on abortion discusses it and Jeff McMahan discusses it. David Oderberg discusses the argument. I could go on and provide a detailed list with references of where this argument has been raised or discussed if you wish.
As to your point about DNA, some human cancer cells have human DNA they are not human beings. Each cell in my hand has human DNA that does not make the cells in my hand human beings. To be a human being requires more than merely having DNA. The twinning argument raises the question of when an individual organism which can be identified as the same organism which develops into you or me comes into existence
March 23, 2009 at 2:14 pm
Sb: “the idea of arguing that a embryo is not human is amazing given that it is composed of Human DNA.”
I agree, the more people try to justify abortion the more ridiculous their arguments become. An embryo certainly is human.
Matthew is correct however that the issue is not just about DNA, as you have pointed out elsewhere although your finger contains human DNA it is not a human. The issue is around an entire organism with DNA, not just a part of an organism. So although your finger contains human DNA, it is not a human, yet you are. A foetus’ arm is not human, but the foetus is.
March 23, 2009 at 4:01 pm
“I agree, the more people try to justify abortion the more ridiculous their arguments become.”
Well, here’s a new one.
Can you freeze a human being and then bring them back, unharmed? No. But you can do that to an embryo.
March 23, 2009 at 5:41 pm
morsecode
The claim that you can’t “freeze a human being and then bring them back, unharmed” seems to me to only be justified *if* you exclude embryos from the class of humans which you use as the basis for that claim.
hence the argument is circular.
March 23, 2009 at 7:21 pm
morsec0de:
I presume you are joking and just providing another “ridiculous” argument for our amusement, but it is hard to tell from your wording.
In case you are serious, whether an organism can be frozen and thawed unharmed is just affected by physical factors – its structure and size. An embryo is smaller than an adult, obviously. The point is irrelevant.
You may as well argue “Can you fit an adult through the handle of a teacup? No, But you can do that to an embryo.”
March 23, 2009 at 9:08 pm
Does that mean if you don’t split an embryo into twins, you have cut off the potential for a life? If so, don’t we have a moral obligation to twin each and every embryo?
I believe the argument is not specifically that embryos are not human, but that they are not human beings or not persons.
March 24, 2009 at 8:44 am
Joshua:
No, I don’t think we must split every embryo in two! You could extend that argument to say we should split every embryo into thousands, which is technically possible, but ridiculous. So it is wrong to kill a human, but not wrong to not make more humans.
In the same way it is wrong to kill a child, but not wrong to choose not to have sex and avoid producing a child in the first place.
March 24, 2009 at 10:25 am
Yes, that makes sense.
So, say you had two very early stage embryos, and each was the product of seperate fertilisation events. Is it acceptable to merge those two embryos into one, producing what is called a tetragametic chimera? Or have you killed a human, despite no cells being killed in the process?
March 24, 2009 at 10:38 am
Joshua:
I had never considered that before. I am not sure what to think of it. It does occur naturally (in rare cases), so I wouldn’t have a problem with that of course, but whether it is ok to do it artificially would be another question. It would be tampering with nature a lot. But on the other hand, neither embryo technically dies. My initial thought is to steer clear of it for precautionary reasons.
Hmmm, anyone else have some thoughts?
April 4, 2009 at 1:01 pm
I’m with Matt on the twinning argument. Marquis is wrong, Pruss is right.
I don’t share Marquis’s enthusiasm, however, for his argument from cell division. I think it (very probably) equivocates between two distinct concepts of identity, I think it draws on what may be a questionable premise (that one of the two cells resulting from the first cell divisions is actually qualitatively identical to the zygote), and I also think that Marquis’s own “future of value” argument against abortion works against him in the argument from cell division, unless he chooses to beg the question. I’ve explained my reasoning here.