WHY would sentience determine human value? Determine personhood? Determine whether or not a human can be killed in the womb?
Short answer: It does not. Humans have value simply because they are human, not because of some acquired property that they may gain or lose during their lifetimes. If you deny this, it’s difficult to say why objective human rights apply to anyone.
Scott Klusendorf explains this with lucidity, in his SLED acronym of non-essential differences (Size, Level of Development, Environment, Degree of Dependency) between an embryo and a person outside the womb.
L = Level of development: True, embryos and fetuses are less developed than the adults they’ll one day become. But again, why is this relevant? Four year-old girls are less developed than 14 year-old ones. Should older children have more rights than their younger siblings? Some people say that self-awareness makes one human. But if that is true, newborns do not qualify as valuable human beings. Six-week old infants lack the immediate capacity for performing human mental functions, as do the reversibly comatose, the sleeping, and those with Alzheimer’s Disease.
Klusendorf explains it in greater detail here (Mistake#4):
Abortion advocates like Mary Anne Warren claim that a “person” is a living entity with feelings, self-awareness, consciousness, and the ability to interact with his or her environment. Because a human fetus has none of these capabilities, it’s not a person. Warren makes two assumptions here, neither of which she defends. First, she doesn’t say why should anyone accept the idea that there can be such a thing as a human being that is not a human person. What’s the difference? I’ve never met a human that wasn’t a person, have you? Second, even if Warren is correct about the distinction between human being and human person, she fails to tell us why a person must possess self-awareness and consciousness in order to qualify as fully human. In other words, she merely asserts that these traits are necessary for personhood but never says why these alleged value-giving properties are value-giving in the first place.
In his article “Why Libertarians Should be Pro-Choice Regarding Abortion,” Libertarian philosopher Jan Narveson makes points similar to Warren. His larger purpose is to tell us who is and is not a subject of libertarian rights. He argues that humans have value (and hence, rights) not in virtue of the kind of thing they are (members of a natural kind or species), but only because of an acquired property, in this case, the immediate capacity to make conscious, deliberate choices. Because fetuses lack this acquired property, they have no rights. A woman’s choice to abort, then, does not negatively effect the fetus or deny it any fundamental liberties.
But this can’t be right. Newborns, like fetuses, lack the immediate capacity to make conscious, deliberate choices, so what’s wrong with infanticide? What principled reason can Narveson give for saying, “No, you can’t do that?”
Peter Singer in Practical Ethics bites the bullet and says there is none, that arguments used to justify abortion work equally well to justify infanticide. Abortion-advocates Michael Tooley and Mary Anne Warren agree. For example, if the immediate capacity for self-consciousness makes one valuable as a subject of rights, and newborns like fetuses lack that immediate capacity, it follows that fetuses and newborns are both disqualified. You can’t draw an arbitrary line at birth and spare newborns. Hence, infanticide, like abortion, is morally permissible.
…In short, IF humans have value only because of some acquired property like skin color or self-consciousness and not in virtue of the kind of thing they are, then it follows that since these acquired properties come in varying degrees, basic human rights come in varying degrees. Do we really want to say that those with more self-consciousness are more human (and valuable) than those with less? As Lee and George point out, this relegates the proposition that all men are created equal to the ash heap of history.
Philosophically, it’s far more reasonable to argue that although humans differ immensely with respect to talents, accomplishments, and degrees of development, they are nonetheless equal because they share a common human nature. Humans have value simply because they are human, not because of some acquired property that they may gain or lose during their lifetimes. If you deny this, it’s difficult to say why objective human rights apply to anyone.