Sean Carroll at Cosmic Variance:
If someone believes that abortion really is murder, talk of the reproductive freedom of the mother isn’t going to carry much weight — nobody has the right to murder another person. Supporters of abortion rights don’t say “No, this is one case where murder is completely justified.” Rather, they say “No, the fetus is not a person, so abortion is not murder.” The crucial question (I know, this is not exactly an astonishing new insight) is whether a fetus is really a person.
I have nothing original to add to the debate over when “personhood” begins. But there is something to say about how we decide questions like that. And it takes us directly back to the previous discussion about marriage and fundamental physics. The upshot of which is: how you think about the universe, how you conceptualize the natural world around us, obviously is going to have an enormous impact on how you decide questions like “When does personhood begin?”
In a pre-scientific world, life was — quite understandably — thought of as something intrinsically different from non-life. This view could be taken to different extremes; Plato gave voice to one popular tradition, by claiming that the human soul was a distinct, incorporeal entity that actually occupied a human body. These days we know a lot more than they did back then.
More here.