Is the human fetus a person with the appropriate rights that follow or is the human fetus something less – a nonperson – and therefore not entitled to the inherent rights due a person? I argue for the former and against the latter to defend the position that the unborn human embryo is a full person at the moment of conception and should be afforded the full rights due human beings by their very essence.
THE QUESTION OF THE PERSON & ABORTION
Professor Millard Erickson phrases the question this way:
One other issue with far-reaching implications, particularly for ethics, concerns the status of the unborn or, more specifically, of the fetus still in the mother’s uterus. Is the fetus to be regarded as human, or merely as a mass of tissue within the mother’s body? If the former, abortion is indeed the taking of a human life and has serious moral consequences. If the latter, abortion is simply a surgical procedure involving the removal of an unwanted growth like a cyst or a tumor. 
What is abortion? Abortion is the intended act of ending a pregnancy, thus ending the life of the unborn child. If abortion is murder, then, in the words of Dr. Jerôme Lejeune, the world renown geneticist who discovered the cause of Down’s syndrome, “It kills a member of our species.”  Human life is inherently valuable and if the unborn are people, which I argue they are, human life is at stake in this debate. Prominent teacher R.C. Sproul throws down the gauntlet in explaining the urgent need for clarity in the abortion debate:
If abortion-on-demand is evil, no one has the moral right to choose it. If it is an offense against life, the government must not permit it. The day is being captured by the moderate middle who have not faced the ethical implications of this position. This is the moral cop-out of our day … Pro-choice is pro-abortion. Be clear about that and abandon the muddled middle. 
These strong words serve as a needed call for consistency as we discuss the issue of abortion and personhood. What then are some common arguments to justify the idea that the unborn are not persons? I have broken the arguments down into four different categories (there are more but I have selected the ones I see most):
1. Sentience makes a person and the unborn are not sentient
2. Size makes a person and the unborn are too small
3. Viability makes a person and the unborn are not viable on their own
4. Wantedness makes a person and the unborn are not wanted
I apologize for the semi-crass way these arguments are stated but for the sake of clarity, the arguments have been oversimplified here. I am neither a scientist nor a philosopher but others who are have done some detailed and salient work on the question of the nature and status of the fetus. Therefore, I will now unpack one of the arguments in more detail, using both science and philosophy as my main tools.
THE ARGUMENT FROM SENTIENCE
Sentience is consciousness; a capacity for feeling or perceiving. One part of being a sentient being is the ability to feel pain. Jim Lippard, an atheist friend of mine, communicated this position to me in a Facebook conversation when he said,
“A two-celled zygote has no brain, no neurons, no mental functioning, no consciousness, no capacity for pain, and is therefore not a person. Personhood is the status required for right to life, therefore zygotes do not have a right to life.” 
A component of this argument is it implies the pro-life position is weak because abortion is not cruel because the fetus cannot feel pain. Does this mean if I am unconscious or sleeping, I have lost my personhood? Ethicist Scott Rae has some good insight on this issue when he says,
“Senteince has little inherent connection to the personhood of a fetus. This decisive moment confuses the experience of harm with the reality of harm. … If I am paralyzed from the waist down and cannot feel pain in my legs, I am still harmed if someone amputates my leg.” 
In addition to this, a baby is sensitive to touch at ten weeks and at eleven weeks, the baby has a skeletal structure, nerves, and circulation. This fact, plus a myriad of experiments and observations lets us know by the third month, the fetus most certainly does feel pain.  Since I do not want to pull a Couric and belabor the point, here is a balanced article from the New York Times (02/10/08) on fetal pain: The First Ache by Annie Murphy Paul
A FALSE DISTINCTION?
Continuing with this line of reason, it seems like an artificial demarcation to distinguish between “human” and “person” as Phoenix atheist/skeptic Jim Lippard does:
Everybody agrees that an individual human sperm and egg are “human” (though not human beings), but not persons. Everybody agrees that something else has to happen before they become persons. … I don't think the fact that a set of cells has the potential of becoming a person is sufficient to be a person – personhood involves having some kind of capacity for awareness and reflection, and it's that capacity that makes persons valuable and deserving of rights like a right to life. A collection of cells with no capacity for awareness is not a person, even if it may someday become a person. 
This paragraph echoes MIT’s Judith Jarvis Thomson’s developmental view, in which the basic thesis is humans become persons by some ability they acquire and not by the kind of entity they already are.  We should take note that abortion proponents merely assert one must be aware and self-reflective to qualify for personhood and hence, the right to life. Very rarely do they bother to attempt to defend this claim with any rigor. Who says they get to lay out the qualification for personhood? It's not as if there is a strong consensus, anyway. Shouldn't a civilized and ethical society desire to err on the side of life?
Besides, it incorrect for Jim to say that "everybody agrees that something else has to happen before they become persons." The truth of the matter is the something “else” happens between gametogenesis and fertilization, not between fertilization and some selected point of development later on.
The sperm is one part of a male human being that joins together with one part of a female human being (an oocyte, often called the ovum or egg). These two parts, which possess human life in a certain sense, form a new genetically unique individual. Upon fertilization, parts of human beings have been transformed into something different from what they were; this means the sperm and the oocyte cease to exist as they were and a new whole human being is produced.
Robert P. George and Christopher Tollefsen dedicate a large portion of their fantastic book, Embryo, to sufficiently dismantling the developmental view and related arguments. I quote at length one highly significant paragraph:
Developed self-consciousness - or desires or the ability to use language and so on – are arbitrarily selected degrees of development of capacities that all human beings possess in (at least) radical form from the coming into existence of the human being until his or her death. So it cannot be the case that some human beings and not others possess the special kind of value that qualifies an entity as having a basic right to life, by virtue of a certain degree of development. Rather, human beings possess what kind of value, and therefore that right, by virtue of what they are; and all human beings, not just some, and certainly not just those who have advanced sufficiently along the developmental path as to be able immediately (or almost immediately) to exercise their capacities for characteristically human mental functions, possess what kind of value and that right. 
One basic problem to the developmental view is we know everything needed for the person is contained in the embryo. A new being with a unique genetic code has come into existence and all the embryo needs now is time plus nourishment. But nourishment to survive and time to mature is what we all need, so how is the embryo’s essence different in any way than an 80-year old man?
THE PREJUDICE OF "DEVELOPMENTALISM" EXPOSED
The only time we as humans have completed our full development is once we are dead (in one sense, in the strictly physical sense development usually stops between 20-25) and even then we have not actualized our full potential. If the developmental view is correct, we must ask some sobering questions. Libertarian for Life member and former bench research biochemist and biologist at the National Institutes of Health, Dr. Dianne Nutwell, does this:
One should also consider simply the logical – and very real – consequences if a "person" is defined only in terms of the actual exercising of "rational attributes" or of "sentience”. What would this mean for the following list of adult human beings with diminished "rational attributes:" e.g., the mentally ill, the mentally retarded, the depressed elderly, Alzheimer's and Parkinson's patients, drug addicts, alcoholics – and for those with diminished "sentience," e.g., the comatose, patients in a "vegetative state," paraplegics, and other paralyzed and disabled patients, diabetics or other patients with nerve or brain damage, etc.? Would they then be considered as only human beings but not also as human persons? Would that mean that they would not have the same ethical and legal rights and protections as those adult human beings who are considered as persons? Is there really such a "split" between a human being and a human person? 
All of this information taken together demonstrates why it is counterintuitive to distinguish between humans and person. It is an artificial and arbitrary distinction with no scientific grounding. One more reason the human/person distinction is artificial is because I have never met a person who is not a human, nor have I ever met a human who is not a person. Is this even possible?.
THE I.Q. TEST WHERE "D" MEANS DEATH
Part of the argument from sentience derives its primary thrust by arguing from rational ability. If rationality is the factor being looked at in regards as to what makes a person, then are all humans below a certain I.Q. level less than a person? Is it really true that the bigger your brain is, the more of a person you are? How can this be?
As strange as it may be, two real-life advocates for a similar position can be found in none other than the famous Nobel laureates, Crick and Watson. Crick believes
“no newborn infant should be declared human until it has passed certain tests regarding its genetic endowment and that if it fails these test it forfeits the right to live”.[13a]
Watson thinks a child should not be
“declared alive until three days after birth, then all parents could be allowed the choice only a few are given under the present system.” [13b]
Well-known "ethicist" Joseph Fletcher thinks one must have an I.Q. of at least 40 to be a person.  Should full-grown adults still take any similar tests? If you pass, you are allowed to live and if you fail, you forfeit your right to life. Why should only infants have to take these kinds of tests? Furthermore, why did Watson select three days as the deadline (pun intended)? Other “choice” (his words) advocates would disagree.
The most prominent example is Princeton's Peter Singer, who believes all infants should have a kind of 30-day return policy. This is a tongue-in-cheek way of saying Singer advocates a limited form of infanticide. Similar to Jim's argument for abortion [NOTE: I AM NOT SAYING JIM CONDONES INFANTICIDE], Singer believes newborns do not possess the core traits of personhood: "rationality, autonomy and self-consciousness"  ... so "killing a newborn baby is never equivalent to killing a person, that is, a being who wants to go on living."  Furthermore, Singer questions if it is even wrong to take innocent human life in the first place:
[The argument that a fetus is not alive] is a resort to a convenient fiction that turns an evidently living being into one that legally is not alive. Instead of accepting such fictions, we should recognize that the fact that a being is human, and alive, does not in itself tell us whether it is wrong to take that being's life. 
The reason I show all this is to exploit the logical conclusions to some of the arguments for abortion. Singer is simply more consistent than most abortion advocates but honestly, I'd rather the rest of them remain committed to their fundamental inconsistencies.
One thorny issue for them is the problem of the animal kingdom: if mental capacity is used to determine personhood, than an adult chimpanzee has a temporary advantage over a young child and a permanent advantage over the mentally handicapped. Here is one scientist who agrees:
“It would seem to be more inhumane to kill an adult chimpanzee than a newborn baby, since the chimpanzee has greater awareness.” 
In a debate in Slate, Richard Posner wrote Singer fails to realize the
"radicalism of the ethical vision that powers [his] view on animals, an ethical vision that finds greater value in a healthy pig than in a profoundly retarded child, that commands inflicting a lesser pain on a human being to avert a greater pain to a dog, and that, provided only that a chimpanzee has 1% of the mental ability of a normal human being, would require the sacrifice of the human being to save 101 chimpanzees." 
I agree wholeheartedly!
Interestingly, some folks who are ardent abortion activists (not merely supporters) are also affiliated with People for the Ethical Treatment for Animals (PETA). It seems they tend to have a low view of humans and a high view of animals. Ironic … or consistent? Unfortunately, I think the answer is both.
 Millard J. Erickson, Christian Theology, 2nd ed. (Grand Rapids, Mich.: Baker Book House, 1998), 570.
 Norman L. Geisler and Ralph E. MacKenzie, Roman Catholics and Evangelicals: Agreements and Differences (Grand Rapids, Mich.: Baker Books, 1995), 365.
 R.C. Sproul, Following Christ (Wheaton, Ill: Tyndale House Publishers, 1996), 20.
 Jim Lippard, e-mail sent to the author, August 16, 2009.
 Scott B. Rae, Moral Choices: An Introduction to Ethics (Grand Rapids, Mich.: Zondervan, 1995), 132.
 For documentation of fetal pain, see Curt Young, The Least of These: What Everyone Should Know About Abortion (Chicago, Ill.: Moody Press, 1989), 78-79.
 Jim Lippard, e-mail sent to the author, August 16, 2009.
 Judith Jarvis Thomson, “Abortion,” Boston Review, 1995.
 Robert P. George and Christopher Tollefsen, Embryo (New York: Random House, 2008), 145.
 Dr. Dianne Nutwell, “When Do Human Beings Begin? Scientific Myths and Scientific Facts,” Libertarians for Life,
 Jim told me about a recent movie, District 9, which seems to imply if we encounter intelligent aliens, they will be persons but not human.
This is similar to the line of questioning put forth in Scott Klusendorf in “Five Bad Ways to Argue About Abortion”
[13a/b] Cited in C. Everett Koop and Francis Schaeffer, Whatever Happened to the Human Race? rev. ed. (Westchester, Ill.: Crossway Books, 1986), 40.
 Joseph Fletcher, “Indicators of Humanhood: A Tentative Profile of Man,” The Hastings Center Report 2 (Novemer 1972), 1.
 Peter Singer, Practical Ethics (Cambridge, UK: Cambridge University Press, 1997), 169-171.
 Peter Singer FAQ, Princeton University, accessed Dec. 13, 2009.
 Peter Singer, Rethinking Life and Death, 105.
 Winston L. Duke, “The New Biology,” Reason, August 1972. Duke is a nuclear physicist if my memory serves correct.
 Animal Rights - a debate between Peter Singer & Richard Posner
Slate, June 2001. http://www.utilitarian.net/singer/interviews-debates/200106
 This does not mean I think animal cruelty is acceptable in any way. I am just pointing out that when human life is not viewed as unique, we slide down a slippery slope - fast. Singer's ethics are a case of "extreme" consistency. For example, on bestiality, Singer says sexual activities between animals and humans that are harmful should remain illegal but then Singer goes onto write that "sex with animals does not always involve cruelty" and "mutually satisfying activities" of a sexual nature can be "OK" between humans and animals. Translation: Singer thinks zoosexual activity need not be abusive and may be mutually enjoyed. Heavy Petting, Nerve, 2001. http://www.nerve.com/Opinions/Singer/heavyPetting/main.asp