It’s an uncontroversial fact of science that the preborn conceived of a human male and human female are human beings (biologically) from fertilization. But does this mean that just because they’re human beings we can’t kill them? There are times when it’s almost universally accepted that it’s acceptable to kill a human being, such as in self-defense. But what if they are innocent of any wrong-doing deserving of being killed? Science can show us that something is human (e.g. you, me, infants, the preborn, etc.), but it can’t show us whether it’s wrong to kill humans. That’s where philosophy steps in.
What is personhood? That’s the eternal question, and one where pro-choice and pro-life advocates constantly disagree. Almost all pro-life advocates agree that personhood is established at fertilization/conception (the only detractor I’m currently aware of is Bernard Nathanson, who believed that we actually don’t become human until implantation). Pro-choice advocates consistently agree that personhood is established at, or slightly before, birth, but they usually have different definitions of “personhood.”
Some pro-choice advocates who accept the biological humanity of the preborn try to argue that while they’re human, they’re not human beings. However, this is a distinction without a difference. The preborn are human entities, they are human beings. A “being” is just something that exists, and the preborn certainly exist. Even claiming that the unborn are not human in a “morally relevant” sense is an artificial designation. Many people disagree on what makes you a human in a “morally relevant” sense. Some even believe that the unborn have some value as human entities, just not as much value as born human beings. Pro-life philosopher Trent Horn calls this Golden Retriever Reasoning.  So what the argument seems to come down to is the unborn are human, but are they persons?
Pro-choice philosopher David Boonin wrote, “But presenting the rights-based argument against abortion in terms of the claim that the fetus is a person is neither necessary nor illuminating. This is so because the term person is ambiguous...person can be used in a purely normative sense. So understood, the claim that the fetus is a person simply means that the fetus has a right to life. On this construal of the term, any reason for believing that the fetus is a person just is a reason for believing that the fetus has a right to life. The claim that the fetus is a person in this sense plays no substantive role in justifying the claim that it has a right to life, and so there is no reason to consider it as a distinct claim.” 
I agree with Boonin. The problem is that considering someone or something a “person” really doesn’t have much use besides granting that individual a set of rights. After all, our own government (in the United States, where I live) is certainly no authority on personhood. At one time in our history, Africans and women were not considered persons in any meaningful sense of the term. Even today, corporations are considered persons. “Person” is just a legal term used to discriminate against groups of humans.
However, in a book edited by Louis Pojman and Frank Beckwith, they state that arguing from being biologically human, alone, is a result of bad reasoning. They write, “Another popular prolife argument goes something like this: Because the unborn entity is a human being from the moment of conception, and because it is morally wrong in almost all circumstances to kill human beings, therefore, abortion in almost all circumstances is morally wrong. Although the prolifer is certainly correct that the unborn entity is a human being in the genetic sense from the moment of conception, it is not clear from the biological facts alone, without philosophical reflection, that the fetus is a human person and possesses the rights that go with such a status.” 
So as you might imagine, I do disagree with them to a point, since I believe that arguing over personhood isn’t always helpful. Additionally, this statement actually seems to beg the question. They assume that only “persons” have a right to life, but give no reason to suppose this. Isn’t it conceivable that non-persons could also have a right to live? In fact, Don Marquis’ Future-Like-Ours argument does not rest upon whether or not the human embryo/fetus is a “person.” It has a valuable future of experiences like ours that is robbed from them, so it is wrong to kill them like it is wrong to kill us.
The problem is that pro-life and pro-choice advocates believe that abortion should be illegal or legal (respectively), so they will try and come up with a definition of person that justifies their position, that it is wrong or acceptable to kill the preborn. Now, my stance against abortion is not based on biology, on the preborn’s humanity, but on what being human means. So if we must discuss personhood, that’s what I base my personhood stance on. The problem with pro-choice definitions of personhood is that they work equally well in disqualifying people who are born from personhood, humans that would uncontroversially be deemed as a valuable human being, if not for that definition of personhood. Just about every pro-choice argument works equally well to justify infanticide, and then the person making it tries to weasel their way out of it with an ad hoc justification for why infanticide is not morally permissible (e.g. Mary Anne Warren). In fact, many pro-choice writers and advocates use the terms “person” and “human” interchangeably, except when they want to justify abortion. Then they suddenly want to make a distinction between the two.
So this leads us to our question: What is a person? Sixth century philosopher Boethius wrote, “a person is an individual substance with a rational nature.”  Though I would actually take it a step farther and say a person is an entity with a rational, moral nature. C.S. Lewis has told us why a moral nature is essential to us as human beings: The human is both a rational and a moral being. Without a moral nature there would be no true humanity, so those who would abolish the moral law would abolish humanity in the bargain.  He writes, “Either we are rational spirit obliged for ever to obey the absolute values of the Tao, or else we are mere nature to be kneaded and cut into new shapes for the pleasures masters who must, by hypothesis, have no motive but their own ‘natural’ impulses. Only the Tao provides a common human law of action which can over-arch rulers and ruled alike. A dogmatic belief in objective value is necessary to the very idea of a rule which is not tyranny or an obedience which is not slavery.” 
A substance is an entity that maintains its identity through change.  You are the same being now as the embryo you were in the womb, with the same essential nature. Since you are the same substance, if a morally justifiable reason is needed to kill you now, a morally justifiable reason was needed to kill you in the womb.
So we, as humans, are valuable because of our inherent nature as rational, moral agents. What makes us a person is not the current capacities that we can fulfill. What makes us a person is our nature, our inherent capacities to fulfill these functions. These inherent capacities are present from fertilization.
So this raises another question: Who qualifies for personhood? The answer is simple: anyone who has a nature similar to that of us. Those with a rational, moral nature. So being human (that is, belonging to species Homo sapiens, because of the nature that humans have) is a sufficient, but not a necessary, condition for personhood. If supernatural beings exist, like God and angels, then they qualify. Extraterrestrials, like Klingons and Vulcans of Star Trek lore, if they exist, would also qualify if they have a nature as rational, moral agents. Incidentally, this means that if extraterrestrials exist, pro-life advocates would oppose aborting the extraterrestrial preborn, as well.
How about animals? Peter Singer would say that while the preborn are not persons, some animals like dolphins and gorillas are. However, I would disagree. If animals could be shown to share a rational, moral nature like ours, then it would follow that they would be persons and have a right to life, like we have (incidentally, while I don’t believe animals are on par with us, I do believe that they should not be harmed or killed unless to fulfill a legitimate need that humans have). While animals seem to display some sense of morality and rationality, it is a very primitive type. They don’t know they are behaving morally (or at least, what is moral for animals), they just act on instinct. Plus, we don’t hold animals to the same moral standard we hold ourselves. We don’t lock up Yogi for stealing a picnic basket (or ask him to reimburse the camper he stole it from). We don’t lock up a wild animal for killing a human (though granted they are sometimes put to death, this is not out of a sense of justice but to protect humans because the animal has now tasted human blood).
I’ve foreshadowed a couple of arguments against establishing personhood at fertilization. One is the Gradualist position, which Trent calls Golden Retriever Reasoning. Another is the Functionalist position, in which it’s not our inherent capacities but the presently-exercisable capacities that grant personhood. I’ll cover these and two others in my next article.
 Trent starts to talk about this at about 15:40 in the video. Start at about 15:00 in to get the context of the statement.
 David Boonin, A Defense of Abortion, Cambridge University Press, Cambridge, New York, 2003, p. 15.
 Introduction to the Book, from The Abortion Controversy, 2nd Ed., ed. Louis J. Pojman and Frank J. Beckwith, Wadsworth Publishing Company, 1998, p. xiv
 Ancius Manlius Severinus Boethius, Liber de Persona et Duabus Naturis, ch. 3.
 Frank J. Beckwith.
 C.S. Lewis, The Abolition of Man, p. 77.
 ibid., pp. 84-85. Note that when C.S. Lewis speaks of the Tao, he is referring to an objective moral law.