I believe that life does begin at conception, and I don't see any way around it. So, for those who disagree, please explain it to me. (And for Mestemia's standard objection, I am referring here to an individual human life - abiogenesis is wholly irrelevant.
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Speaking for myself, I'm not so sure that "personhood" isn't just a human-created idea. Using what might be an odd analogy, I think it's like a combined living/dining room: where does the living room end and the dining room begin? Well, the living room ends somewhere after the sofa and begins somewhere before the dining room table... but without furniture, you can't really say.
If that makes any sense at all. I'm still tired from the weekend, so I may be talking nonsense.
My main point is that human development and change is a smooth continuum that starts well before birth and only ends when we die. Some parts of that continuum may be more steeply sloped than others, but it's still a judgement call as to where the transition from one state to another, or one valuation to another, occurs.
Also, I don't know if you're doing this, but I see a line of reasoning often employed on this issue that seems to me to be fallacious: we start at birth and assume that the baby is a full human person. Then, we trace back along the development during pregnancy to find a clear demarcation point between "person" and "not person". Only finding a usually-gentle slope, we conclude that the demarcation point isn't there. We then proceed back to conception, which we identify as a demarcation point in certain respects and conclude that it must be the demarcation point for personhood as well.
I think this reasoning is short on support for the idea that conception should be considered the initial point of personhood, and I think it's incorrect in assuming that "personhood" (however it's defined) is something that cannot be developed gradually and incrementally over time.
To use another analogy, think of digging a hole in gravel by removing it a stone at a time: when you have an 8-foot deep pit, you can be sure it's a hole (though is it a "pit"? Probably not). You can be fairly sure it was still a hole when it was only 1 foot deep as well. In fact, you can't really identify any time when it abruptly changes from "not a hole" to "hole", but does this mean that the ground suddenly became a hole when you picked up the first pebble? I'd say it didn't.