By lacking the physical characteristic of personhood, which is __________________.
How do we fill in the blank?
The body, if I had to answer.
I do think that this was a rhetorical question on the part of the Pope, intended to highlight the absurdity of the proposition that a living human, which a fetus certainly is, could be anything other than a human person.
Depending on how we define “individual”, however, we might argue that when a person dies and their soul leaves the body they are, prior to the Resurrection, a human individual while not being a human person. They would be lacking the physical characteristics of human personhood.
I don’t see any way, however, that we could reasonably argue that a living human body, which we now know begins at the very moment of conception, is not a human person. It is biologically alive, indicating the presence and activity of a soul, and is biologically human. One doesn’t even have to believe in an immaterial soul to accept this, as pro-life atheists can attest.
Other related issues, such as the relationship between identical twins, need not come into the picture, nor do they need to raise particular difficulty. If we like we can simply say that a new soul is created when the embryo splits, or that the original dies and two new ones are born. Chimera, when two embryos fuse together in the womb and produce a person with two sets of DNA don’t seem to be a whole lot different than cases of organ donation, at least insofar as ensoulment is concerned.