In the article “Human Non-Person”: Terri Schiavo, bioethics, and our future Wesley Smith explains
If you want to know how it became acceptable to remove tube-supplied food and water from people with profound cognitive disabilities, this exchange brings you to the nub of the Schiavo case — the “first principle,” if you will. Bluntly stated, most bioethicists do not believe that membership in the human species accords any of us intrinsic moral worth. Rather, what matters is whether “a being” or “an organism,” or even a machine, is a “person,” a status achieved by having sufficient cognitive capacities. Those who don’t measure up are denigrated as “non-persons.”
Allen’s perspective is in fact relatively conservative within the mainstream bioethics movement. He is apparently willing to accept that “minimal awareness would support some criterion of personhood” — although he doesn’t say that awareness is determinative. Most of his colleagues are not so reticent. To them, it isn’t sentience per se that matters but rather demonstrable rationality. Thus Peter Singer of Princeton argues that unless an organism is self-aware over time, the entity in question is a non-person. The British academic John Harris, the Sir David Alliance professor of bioethics at the University of Manchester, England, has defined a person as “a creature capable of valuing its own existence.” Other bioethicists argue that the basic threshold of personhood should include the capacity to experience desire. James Hughes, who is more explicitly radical than many bioethicists (or perhaps, just more candid), has gone so far as to assert that people like Terri are “sentient property.”
What the definition of “sufficient cognitive capacities” is remains to be seen. Once people are seen as “killable and harvestable” sentient property, any number of justifications can be found to kill and harvest any number of people. And that could mean you, and me.