Dr. Scott Rae of Talbot School of Theology, an expert in bioethics, describes the differences between a functional view of persons and an ontological view of persons. When it comes to dealing with medical issues like anencephaly (partial or total absence of a brain) and persistent vegetative states (PVS), these distinctions become extremely important.
The severely neurologically impaired, such as the anencephalic newborn, the PVS patient and the nursing home resident at the end stages of Alzheimer’s, raise puzzling questions. They are alive, but do not have much of a life, when it comes to the narrative that distinguishes them from their mere bodily functions. Our intuitions tend to rebel against the notion that they are human beings like us, because they look and seem to us to be simply bodies that medical technology is sustaining.
These cases raise the question: “How much brain do you need to be human?” Or to put it more generally: “What kinds of capacities are necessary for one to be considered a person?” Underlying the former question is another criterion for personhood, that of consciousness/sentience. With the neurologically impaired, the question we are really asking is: “Can someone be a person without being conscious or sentient?”