The first definition--let's call it biolife--is a biological definition. Once an egg is fertilized, the resulting zygote has a determined its complement of genes. It's a profoundly definitive event: you now have determined the genetic identity of a new diploid organism. That same organism will develop, metabolize, possibly reproduce, and eventually dye. It's really interesting biologically.

But it is not what we mean by the second definition--let's call it sociolife--in almost any case. Philosophical literature is full of talk of consciousness, rationality, belief, emotion--all stuff that the zygote doesn't have. If we make any claims about human life being special or unique or tautological or anything, it's certainly not about biolife or we would admit everything from tunicates to mosquitos.

What is interesting to humans about humans (other than that we all share very similar genes) is qualia and consciousness. That's what makes us us. If a friend had a "body transplant" but retained the same conscious identity, you would probably be happy to continue calling them your friend, even if they had different genes.

Now, you could say: well, yes, we find that interesting but it's all really just reflecting our genetic imperative to reproduce. The biolife is what's really important here, and we just rationalize sociolife as what's important because it's so easy to distinguish and because we spend so much of our conscious effort thinking about it.

And that could be a reasonable thing to say, except you then have no basis to value a zygote in cases where the species-level reproductive fitness might actually be reduced rather than increased by having another individual present right now. Animals of all sorts, including humans, avoid blind, rampant procreation when it would be deleterious--or they don't exist any more.

So you can't, as far as I can tell, go that route.

And the sociolife route, while open, doesn't translate to "human life begins at conception", because that sense of human life does not begin at conception. Just because conception is a place where there is some important transition it does not follow that it is the relevant transition.

Now, it is of course possible for individuals to hold views on the basis of incomplete analysis, and I do agree that the biolife/sociolife distinction is easy to gloss over (especially given the linguistic forms used to talk about it). So it may be that a secular pro-life position is most commonly the result of just such a confusion. That it is a confusion or illogical doesn't mean it's not what happens, just as most people believe they are above average.

(And, to boot, that they are better at estimating how average they are than the average person. Even when you tell them that people believe they are above average. It's really hard to get an honest self-evaluation out of a person.)

So if you're talking about logically valid reasons to be secular and pro-life, there isn't much room. If you're talking about instincts or sloppy reasoning ("I value babies; I know there's no sharp transition anywhere from zygote to baby; so I value zygotes just as much"), then sure, there's plenty of room.

The quasi-reasonable secular reasons tend to be much more complicated and sound more like reasons why noise ordinances are a good idea, or why children shouldn't be allowed to play violent video games-, or why homosexuality should be outlawed. Hopefully this further clarifies things, @yters.

@RexKerr The fundamental point is the only sharp distinction in life is the point of conception. After that everything is a continuum of various levels of development. And all states of development you'd use to distinguish a zygote from an adult end up justifying killings of various other impaired, underdeveloped or even merely comatose adults. That is why the only logically consistent position is personhood from conception.

@RexKerr Now, I'd consider abandoning personhood at conception if you listed independent characteristics (i.e. not not a zygote) that would only make it ethical to kill a human at a zygote stage but no other stage. For example, undeveloped is not a principled reason. You have to qualify development by stage, which in turn reduces to the question begging not-a-zygote characteristic.

@yters - When should you flee a house where something has caught on fire instead of trying to put it out yourself? The only sharp distinction is when anything in the house is burning that shouldn't be. When is there a "pile" of sand?--there are no sharp distinctions except that one grain of sand is sharply different than zero. That a distinction is sharp does not make it relevant.

Let's consider a plate of tissue culture cells. They're certainly alive and packed with information that allows them to propagate. They can't do so outside tissue culture media, but then regular people can't live outside an environment close to standard temperature and pressure. There's no clear distinction there, either, except that a cell is very different than no cells.

I don't think one can support that only a zygote can be killed but not a blastula because there isn't any relevant difference. Neither has any functioning nervous system, which means neither can experience anything (in the sense that we mean "experience").

But a newborn is very different: huge complex nervous system, all sorts of (relatively unrefined) behaviors; it clearly does experience things, if not exactly the way we do. Somewhere along the way things have changed (and we have a pretty good idea of what changes when and how much).

I am not well-enough versed in human development to propose any particular threshold for how much of the full sanctity of human life an embryo or fetus has picked up, but neither ignorance nor a desire to avoid messy yet relevant transitions in favor of an irrelevant but sharp boundary makes for a good argument.

Also, I don't know why you state that "undeveloped is not a principled reason". An undeveloped painting is not treated the same as a completed one. An undeveloped idea isn't either. Nor is an undeveloped plot of land. Or an undeveloped friendship. Or an undeveloped chicken egg. Development matters hugely in our attitude towards all sorts of things. Is your argument that it doesn't simply, "I am afraid of a slippery slope"? That's all I can see.