Edward asked me a perfectly reasonable question, probably the most common question I get when I talk about my views on abortion: how could a fetus conceived by two humans not be human? My answer got a bit long for a comment, so I thought I'd make it a post. Besides, I thought some people might find it interesting.
I have no problem saying a fetus is genetically human - that it has the genetic code of a human. If that's all it takes to be a human, then I suppose in that sense Edward is right and the offspring of two humans has to be a human. But my understanding of life science --and any scientists, please correct me!-- is that species aren't just determined by their genetic code. Organisms have a structure, an arrangement of cells. After all, I got my hair cut yesterday and shed nary a tear over the mass genocide of split ends. And we don't drag doctors before the review board when they excise a cancer and those human cells die, because cells aren't humans.
That's an intuition I think we all have - that a human involves not just a certain DNA but also a certain structure and (dare I say) a certain set of capabilities. A zygote in the earliest weeks of a pregnancy is a clump of cells. There's no structure, let alone no characteristic functions of being human. So that clump of cells isn't a human, though it may have human DNA.
Our language supports this conclusion. An acorn is not an oak tree, though it comes from an oak tree and will grow into one. Neither is a tadpole a frog in any obvious sense. It lacks critical abilities like the capacity to breathe air rather than air, the existence of legs for jumping, and the like. With humans we tend to use the same word for all stages of development, but I think this is a bit of a misnomer. Or at least it lends itself to misuse.
Here's where things get tricky. When people say abortion is murder, they usually have an argument in mind along the lines of:
1. It's always wrong to kill a human (setting aside self-defense, accidents, etc.)
2. A fetus is a human.
3. So it's always wrong to kill a fetus (setting aside self-defense, accidents, etc.)
Problem is, people are using the word "human" in very different ways here. I'll grant that (1) is true if we're talking about an adult human who's able to think and evaluate the situation - a rational animal, in Aristotle's terminology. I'm even willing to extend that to small children who aren't yet fully rational but are on their way, and to older fetuses that can react to their environments and show signs of self-awareness, decision-making, etc. But a blastocyst can't think.
On the other hand, it's only obvious that (2) is true if we're talking about a genetic human. Young fetuses --before consciousness-- are only human if we understand human in a very different way here than we did in statement (1). Ergo: equivocation. To avoid that, I have trained myself to only use the term "human" in the first sense (a self-aware being, mainly).
Now, Christians (myself included) may want to talk about the soul as well. One definition of human is something that has a soul. That is wonderfully unhelpful to my philosopher's mind because you haven't explained what a soul is, and what reason you have for thinking that humans have one or that only humans have one (making the killing of a human worse than the killing of an ox). But I'll set aside those issues for the moment. What evidence do we have that the soul enters the body at conception, or implantation, or whenever? I have a vague memory of one of the Catholic saints who said that the soul joined the body at quickening - Innocent III, maybe? I remember Gregory VI (1500s) issued a bull clarifying that abortion was only post-quickening, and that that changed the position of an earlier pope who said abortion was at any point in the pregnancy. The idea that death pre-quickening was an abortion was a bit of an aberration at that point, IIRC. So if the soul doesn't enter in right away, then until that happens the fetus is only a potential human, not a full human - even though it has human DNA.
Anyway, those are my thoughts. And yours? Feel free to discuss in the comments.