Assuming I understand who you are referring to when you say "conception right to lifers", I would argue that their position is the most arbitrary of all.
Why? Define the exact moment of conception, and then we can discuss it.
Well there we have to disagree, I believe that theirs is the least arbitrary. The moment of conception a new human DNA sequence is created, uniquely human. It is simply the least formed state of a human but it is not zebra, gecko, elephant nor plant. Their belief is that you protect the lifes of humans no matter the stage of development. as was so aptly stated earlier, a 1 second old baby is not fully developed nor is a 20 year old. To the purist right to lifer you protect human life no matter the stage. 90 years old and on a respirator in a persistent vegetative state protected, born with out a brain protected, 1 second after conception protected, cerebral palsy protected, retarded with an IQ of 10 protected. I don't agree with their standard but they believe a unique human begins at conception. PS as does science.
Yes, I understand where forced birthers draw the line - I've yet to meet a real live purist though.
I think you misunderstood my question. "Define the moment" = when exactly does conception occur? You've presented a philosophical view, but that's not what I asked.
And for the record, feel free to expand n your PS remark - what do you mean "unique human being begins at conception"? As best I can tell, you're saying that every human is unique - I agree but don't see the relevance, nor are you presenting much of anything when you say "as does science", as if "science" is an actual entity with awareness and opinions. People express opinions, fields of study do not.