The question that defines the issue for me is:
What is it that makes a person different from any other animal, and worth preserving above other animals?
I think the answer you have given is the Law. When questioned further about what makes Law right - in other words, not just a bunch of cultural rules that we live by, you reference a fundamental Truth of some sort.
Law (where you derive your literal legal definition) must be based on something. Either it's cultural and subject to interpretation by the standards of the society or it represents some sort of fundamental moral Truth. Right?
If it's cultural and represents no higher moral truth - then it's arbritrary, subject to society's whims. More then that, it means that there is nothing special about human life that makes it more valuable then any other animal.
If it does indeed reflect a higher moral Truth then we have overstepped the bounds of hard science and we have to use the language of religion or philosophy to understand it because science does not have the necessary vocabulary.
My feelings - my beliefs - are that there is something in a person that seperates him from other animals. I call that "personhood" - another might call it Consciousness, Nefesh or Soul.
The difficulty I have with your argument is you are basing it on science (ok - we can establish that a blastocyst is without a doubt a human being - of the species homosapiens) and the letter of the law. But the rightness of the law comes from ... what? I am thinking that that "what" is what also defines a "person" and seperates us from other animals. Because of these questions - how can I accept your argument based on the literal word of the law as an argument when it could just be arbritrary, different in different societies? Accepting it means that there is nothing inherently more valuable about human lives then say, chicken lives.
