They confuse human value with human function.
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 anyone should accept the idea that there can be such a thing as a human being that is not a human person. What's the difference? Have you ever met a human who wasn't a person? Second, even if Warren is correct about the distinction between a human being and a 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. According to Narveson, because fetuses lack this acquired property, they have no rights. Therefore, a woman's choice to abort 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 disqualified. You cannot draw an arbitrary line at birth and spare newborns. Following Narveson's logic, infanticide, like abortion, would be morally permissible.
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.
Natural Rights Versus Legal (Positive) Rights
Put differently, pro-life advocates, echoing Lincoln, argue that we must distinguish between natural rights and legal ones. Natural rights are those rights you have simply because you are human. They are grounded in your human nature and you have them from the moment you begin to exist. For example, you have a natural right not to be harmed without justification as well as a natural right not to be convicted of a crime without a fair trial. Government does not grant these basic rights. Rather, government's role is to protect them.
In contrast, legal (or positive) rights are those rights you can only acquire through accomplishment or maturity. These rights originate from the government. They include the right to vote at your eighteenth birthday, and a right to drive on your sixteenth.
But your natural right to live was there all along. It came to be when you came to be.
To further illustrate, if you are an American, you do not have a legal right to vote in the next Canadian election for the simple reason that you are not a Canadian citizen. But just because you lack the right to vote in Canada does not mean that you lack the right to basic protections whenever you visit that country. Likewise, just because a fetus may not have the positive right to drive a car or vote in the next election does not mean he lacks the natural right not be harmed without justification.
Elective abortion unjustly robs the unborn of his or her natural right to life. In Natural Rights and the Right to Choose Hadley Arkes explains: "No one would suggest that a fetus could have a claim to fill the Chair of Logic at one of our universities; and we would not wish quite yet to seek its advice on anything important; and we should probably not regard him as eligible to vote in any state other than Massachusetts. All of these rights and privileges would be inappropriate to the condition or attributes of the fetus. But nothing that renders him unqualified for these special rights would diminish in any way the most elementary right that could be claimed for any human being, or even for an animal: the right not to be killed without the rendering of reasons that satisfy the strict standards of justification."
Do Women Have a Natural (Fundamental) Right to Abort?
Secular liberals insist that abortion is a fundamental human right the State should not infringe upon. Borrowing a question from Hadley Arkes, we ask, "Where did that right to abortion come from?" In other words, is it a natural right that springs from our nature as human beings, or is it a legal (positive) right granted by the government? If the latter, the abortion advocate cannot really complain that she is wronged if the State does not permit her to abort. After all, the same government that grants rights can take them away.
On the other hand, if the right to an abortion is a natural right---a right one has in virtue of being human---then the abortion advocate had that right from the moment that she came to be, that is, from conception! Thus, we are left with this amusing paradox: According to the logic of many abortion advocates, unborn women do not have the right to life while in the womb, but they do have a right to an abortion! In short, liberals cannot tell us where rights come from or why anyone should have them. As Arkes points out, they have talked themselves out of the very natural rights upon which their freedoms are built.
Proceed to the Mistake #5