Domenico Bettenelli has a post about a Vatican conference on the issues raised by research involving stem cells derived from embryonic human beings (linked above).
The article he cites, at Catholic World News, observes that the Catholic Church's opposition to such research "is now under attack, the Italian bishop noted, with advocates of embryonic research and of the “morning-after” pill advancing the argument that human life does not begin until the fetus is implanted in the uterus" (emphasis added).
Now, let's think about this -- not theo-logically, but merely logically.
Shall we say that "being human" depends on this exterior interaction -- i.e., the fetus being implanted in the uterus?"
Consider: what if a human embryo could mature fully without ever being implanted in a woman's uterus? Not too many years ago, that would have sounded rather fantastic, but is it any longer? Let's not kid ourselves -- someone has to be working on this -- an artificial womb.
So, shall we really say, "this? This is human, because she or he was implanted in a uterus; but here? No; because it never was."
No theology here; just basic reason. Is that reasonable? Someone defend this, please.
Because what that means is that being human is not an intrinsic quality -- but an extrinsic quality. And if that's true -- if it's something "added from outside," then others -- not the putative human subject we're talking about -- decides.
And, of course, that's where we've been with legal abortion thus far. The actual legal reasoning at work in our present law on the subject is that location makes one a person -- specifically, the location of your head: if the fetus' head is located inside the womb, he or she is not a person. Hence partial-birth abortion.
Those of us who live in societies with the expectation of safeguarding that gift of Christian civilization -- universal human rights -- live every day in the expectation that our humanity is not in question; we don't have to prove it; we don't consider that anything exterior to ourselves would make, or unmake, our humanity.
Try thinking about life without that...go ahead: imagine living in a world in which whether you are human depends on what someone else says about you; or what someone did -- or didn't do -- to you at some stage of your development. Two "beings" are in all ways essentially the same; one is human because of where he gestated; the other is not.