...the whole is greater than the sum of its parts.
What does it mean to be human?
Must one have two arms, two legs, an IQ above 150, or what?
The fact is, while many humans do have two arms and two legs, and some do indeed have an IQ above 150, this is not what defines a human. A human is a being made in the image of God. That's why a human without an esophagus is still a human, why a human snoozing in a womb is still a human, a human depressed by life is still a human, and why a respirator-dependent human is still a human.
It's a spiritual thing, not a physical thing.
That's why any purely physical definition will always fall short.