He's a ethicist at Princeton. Arguing personhood is the key factor makes for some messy conclusions. He's come under fire for his views, and they've from time to time been misrepresented. I quote them below.
However, it should be pointed out that the difference between a fetus and a brand new newborn is, in fact, the source of their oxygen and a few other fairly trivial physiological functions. Note that the term necessary for viability, the old Roe standard, has so changed that fetuses that were once a lost cause are now fairly routinely saved as children.
In the absence of a definition of 'person', birth is a fairly important social idea; and that's where Singer, ultimately I think, takes it. But in terms of mental awareness and cognition, how most of us would define 'person' not so much: note the reasoning for Terri Schiavo, in which she was still biologically a person, and even DUers granted her a name. But it takes a while for the visual centers to even sort themselves out, and a number of other autonomic functions are a bit shakey at first. Higher cognition--on the order of what a mouse can do--takes longer. Until months after birth.
I think Wills argument gets him part of where he wants to go: to argue that abortion's not killing a person. But it becomes fairly hard to argue that a 4-month-old baby is a person, on the same terms.
http://www.princeton.edu/~psinger/faq.html========
Q. You have been quoted as saying: "Killing a defective infant is not morally equivalent to killing a person. Sometimes it is not wrong at all." Is that quote accurate?
A. It is accurate, but can be misleading if read without an understanding of what I mean by the term “person” (which is discussed in Practical Ethics, from which that quotation is taken). I use the term "person" to refer to a being who is capable of anticipating the future, of having wants and desires for the future. As I have said in answer to the previous question, I think that it is generally a greater wrong to kill such a being than it is to kill a being that has no sense of existing over time. Newborn human babies have no sense of their own existence over time. So killing a newborn baby is never equivalent to killing a person, that is, a being who wants to go on living. That doesn’t mean that it is not almost always a terrible thing to do. It is, but that is because most infants are loved and cherished by their parents, and to kill an infant is usually to do a great wrong to its parents.
Sometimes, perhaps because the baby has a serious disability, parents think it better that their newborn infant should die. Many doctors will accept their wishes, to the extent of not giving the baby life-supporting medical treatment. That will often ensure that the baby dies. My view is different from this, only to the extent that if a decision is taken, by the parents and doctors, that it is better that a baby should die, I believe it should be possible to carry out that decision, not only by withholding or withdrawing life-support – which can lead to the baby dying slowly from dehydration or from an infection - but also by taking active steps to end the baby’s life swiftly and humanely.
Q. What about a normal baby? Doesn’t your theory of personhood imply that parents can kill a healthy, normal baby that they do not want, because it has no sense of the future?
A. Most parents, fortunately, love their children and would be horrified by the idea of killing it. And that’s a good thing, of course. We want to encourage parents to care for their children, and help them to do so. Moreover, although a normal newborn baby has no sense of the future, and therefore is not a person, that does not mean that it is all right to kill such a baby. It only means that the wrong done to the infant is not as great as the wrong that would be done to a person who was killed. But in our society there are many couples who would be very happy to love and care for that child. Hence even if the parents do not want their own child, it would be wrong to kill it.
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