Humanity isn’t defined by someone’s mind. Plenty of animals are conscious creatures with pretty advanced brains, who think and are curious and “love” and act. Perhaps there is a point at which an animal’s brain becomes hyper-intelligent enough to resemble humans, a point that none of them reach, but aren’t there humans out there with significant brain damage? Why not treat their ‘inferior’ minds like that of an animal?
Humanity is defined by something else:
Genesis 9
6 Whoso sheddeth man’s blood, by man shall his blood be shed: for in the image of God made he man.
Sacred vessels. That’s the short and thick of it. Athena has her owls, Artemis has her deer, and God has His humans. You can say that doesn’t apply to fetuses because they’re underdeveloped, but then you’d have to apply the same logic to children, too.
Humans intuitively know that killing their fellow humans is wrong. There’s the ensoulment aspect, to be sure, and the aspect of reaping the soul of your own child prematurely. But the basic moral is that humans are untouchable when it comes to killing them, unless they themselves cross certain lines and become “bloodguilty.”
Of course, this is a religious commandment–just like all law, really. The Pro-Life movement is essentially doomed, if it tries to enact this law through anything but Theocracy. And that means far more than saying “don’t kill embryos.”
Your argument only works for those who already believe.
For those who don’t, for example because the idea of omnibenevolence could be alien but doesn’t look like what religion teaches, a different mechanism for discerning personhood must be created, for humans, their successors, and potential aliens.
And the truth is that if you remove everything but the brain stem, about the greatest brain damage that the body might maybe survive, what you have isn’t a person anymore, but just an empty shell, and people would realize that. It can be dangerous to violate those rules and boundaries for other reasons, secondary effects and worries about ourselves. This doesn’t mean our intuition is correct all the time - humanity’s intuitions aren’t even unified.
You know darn well that if someone makes an animal-level moral classification for some subset of humans, some asshole will come along and try to push as many people as possible into that classification regardless of whether they are even remotely brain damaged. We already have special legal categories that recognize reduced agency.
