The reason religion is so powerful

[Belacqua]

I know this question wasn't directed at me, but: To the same extent that I know a rabbit doesn't have full personhood because its brain isn't capable of undertaking person-level activities, I know a fetus with fewer brain cells than a rabbit isn't yet a person.

I understand that this is used in practice as a standard. The idea that abortion is OK up to a certain point but not after is based on this, I think. It's certainly a more defensible option than saying that the thing is a fetus and not a baby up to the point of birth.

While in the womb, I'm not sure the fetus is using its brain at person-level activities, even quite late. After a certain point, it has the potential to do so after it's born. 

Is there an objective way to define "person-level activities"? Is it language? Rational thought? How do we define this non-arbitrarily? 

 The only way to argue that a fetus is already fully a person is to bring in supernatural qualities that can only be claimed, not detected. If justifed belief is knowledge, my position has that justification while the opposite position is based on nothing that can be verified at all.

Here I think maybe you haven't read the thread yet. No supernatural arguments have been offered. 

Your position seems reasonable in practice but comes into play at a fairly arbitrary point in the life cycle. It also demands that we define "person-level activity" and that isn't unproblematic. Who decides?

The "opposite" position, if that is what Neo and John have been arguing, is certainly based on something that can be verified: fertilization. Their argument depends on potentialities. Fertilization is the point at which the ontological change occurs. From that point potential is developed along a very subtle sliding scale. Development continues throughout life and ends at death. These are verifiable facts. 

To them, a person begins when a discrete organism with the required genetic material begins its development. For you, a person begins at some point along the sliding scale of that development, which we decide on. "Human-like activities" appear at a point when we define them as doing so, depending on a non-provable definition of that term. 

The "opposite" position wishes to avoid using non-provable definitions changeable by governments. It uses the point at which the discrete organism begins its development -- the non-arbitrary point at which a new thing may be said to exist.