The problem is twofold. First, it's incredibly ambiguous. "Life" can mean a whole bunch of things. I believe when pro-life people use the term, they mean it in the sense of "continuity of identity". To them, the single cell (zygote) is equivalent to a person that you see walking down the street. It's an interesting question of identity that I'll address later. The real problem is that life can also just mean "something which is living". No one can deny that the zygote is alive - that's an objective, scientific fact. The problem is that it's only alive in the same sense that the rest of our cells are alive, or in the sense that a cat, cow, or mosquito is alive. Clearly living doesn't confer any special benefits to any entity. What counts is personhood. "Personhood begins at conception" is a much worse slogan though.
The second problem is a bit more serious. If we take the clarified phrase "Personhood begins at conception" to be the intended meaning, what do we do with all the weird complicating issues?
Take identical twins, for example. They both come from the same zygote, and share the same DNA, but we consider them to be different people. There are a couple of different ways that you can deal with this, and still maintain that personhood starts at conception.
- Identical siblings are actually just one person. This is logically consistent, but you'd get laughed at by nearly anyone you said that to.
- Identical siblings follow a special rule for personhood wherein they don't become people until the split happens. The original zygote was probably not considered to be a person in its own right. This is a poor explanation, because it's a partial concession that there are other factors for determining when personhood begins. Needless to say, "Personhood begins at conception, unless you're an identical sibling in which case it begins at division" makes the slogan pretty much unusable.
- A zygote which will eventually become identical siblings was actually two (or more) people all along, we just didn't know it. This is sort of logically consistent, but it defies the whole continuity of personhood argument a little bit.
Here's weird complicating issue number two: chimeras. What happens with a chimera is that two different zygotes (from two different sperm/egg combinations) fuse together, and then that combination goes on to make one person who has some body parts made from one set of genetic code and some body parts made from the other. Here are some solutions:
- A chimera is actually two people. This means that legally, marrying one would be considered polygamy and murdering one would count as two homicides. This is stupid, and I doubt anyone would be content with this logic.
- Chimeras follow a special rule for personhood whereby they don't count as people until the merge happens. This has the same problem as with twins above, where it doesn't make all that much sense.
- The two zygotes that make up a chimera were each only half of a person. But that wouldn't make sense, because it would be a tacit admission that not every zygote is a full person, and introduces the whole concept of "personhood calculus" into the mix, which most people are quite eager to avoid. (I am too, mostly because the results of personhood calculus leads to things which are politically incorrect, but which I still believe to be true.)
All this is leaving aside the fact all of these concepts like "life", "personhood", etc. aren't clearly defined in the first place. It's sort of puzzling to me that the legal definition of "person" is so murky - but then again, not really, because there are so many corner cases that making a clear distinction on what's what is just too damn hard (see above). From everything I've read about law, this is a bug inherent in legal systems.
It's usually at this point that someone would ask me, "Well, if personhood doesn't begin at conception, then when *does* it begin?" Unfortunately, I don't know, mostly because it's a problem with really poorly defined subjective terms. It's my feeling that personhood would be better understood as a gradient rather than a binary, but that raises a whole host of other issues, most important of which is how you determine where an entity sits on that gradient. I should also note that I'm including in these thoughts things that are more exotic than the timeline of human reproduction: animal consciousness, artificial intelligence, genetically modified humans, extraterrestrials, proto-humans, etc., the idea being that a proper theory of personhood would be all encompassing. I'll let you know if I ever figure out a system that works without kinks, but don't count on that being forthcoming anytime soon.
All of this is basically my way of saying that anytime you see a slogan being thrown around about a contentious political issue, you should immediately start thinking about the ways in which they're simplifying a complex issue. Due to my unbounded faith in humanity, I think most of the time they don't mean to be deceptive, they just haven't given things much thought.
(A word on souls: it's the position of some (ex. the Catholic Church) that the thing which defines personhood is the presence of a soul. Leaving aside how unscientific that idea is, this still leaves pretty much all of the same issues, and raises a few more. Do the two zygotes which form a chimera each have half a soul? Do twins only have half a soul each? Are souls imparted to zygotes which (it's debatable) are not human by virtue of chromosomal disorders? If the soul is immaterial, how can we possibly answer these questions? What reason is there to think that the soul isn't imparted at birth, but at quickening or when the fetus was "formed" (which was the position of many people, including the Catholic Church, for centuries)? Again, this is leaving aside the fact that I think the concept of a soul is incredibly stupid.)