This is a good point. And now that you mentioned it, I didn’t give it enough attention. Just because there isn’t an airtight method doesn’t mean there isn’t any method at all.

I partially addressed this in Part 6 of the series, differentiating between an entity that is entirely inside of a person and one that is not, which is one way to distinguish a fetus from a person. But there are surely additional criteria of personhood.

Thanks for the addition.