Now that we’ve learned what goes into making an argument, let’s talk about how to respond to an argument. First, a bit about why philosophy is necessary.
Philosophy is all around us. Why are you an Atheist/Christian/Buddhist/
Consider my argument from my last article,
P1: It is prima facie immoral to kill a human being.
P2: Abortion kills a human being.
C1: Therefore, abortion is prima facie immoral.
P3: The unjustified killing of human beings should be illegal.
C2 (from P2 and P3): Therefore, abortion should be illegal.
First, I will justify my argument. Then I will illustrate how one could respond to it.
Premise One: It is prima facie immoral to kill a human being.
Prima facie is Latin for “at first sight.” I include this because it is not always immoral to kill a human being. Some forms of killing are justifiable. For example, self-defense. Every human being has a right to self-defense. If someone wishes to take your life, you have the right to defend yourself (within limits). The right to self-defense also entails reasonable measures being taken. If your life is not being threatened, you do not have the right to take life.
Human beings are uniquely valuable based on the kind of thing they are, humans (that is, members of species Homo Sapiens). I do not mean that humans are valuable simply for being human (as opposed to being a dog, cat, etc.). But human beings are valuable due to their inherent nature as rational, moral agents.
The burden of proof must always be on the one wishing to take human life. We do not allow anyone to take anyone’s life without proper justification. Murder is illegal, as are infanticide, genocide, etc. If you look at every human being, we all have different hair color, eye color, height, weight, skin color, etc. The only thing we all have in common that makes it as wrong to kill one human as to kill another is our common humanity. Ergo, it is unjustified to kill someone without provocation.
Premise Two: Abortion kills a human being.
This is self-explanatory. Science has proven long ago that the unborn are living human beings (biologically). The only people who disagree with this premise are lay pro-choice people with an agenda.
The unborn from fertilization are alive because they grow. They also exhibit other forms of life, such as cell division, metabolism, and response to stimuli. In fact, the only thing the unborn need to survive are adequate nutrition, a proper environment, and an absence of fatal threats. That’s all any of us need. There is no point in human development when the developing entity goes from non-life to living.
The unborn are also human from fertilization. We know that everything reproduces after its own kind; dogs have dogs, cats have cats, and humans have humans. They have separate human DNA from, and often a different blood type than, the mother. A white human embryo can be created in a petri dish, implanted into a black mother, and be born white. In fact, if the unborn organism were simply a “part of the mother’s body,” then the pregnant woman would have four arms, four legs, two heads, four eyes, two noses, and roughly have the time male reproductive organs. But this is absurd. At no time during human development does the unborn ever go from “non-human” to human.
In every scientific sense of the word, the unborn is a separate, whole, living, human individual organism. I have literally dozens of quotes from embryologists, doctors, and pro-choice philosophers that attest to this I could offer, but in the interest of space, I’ll offer two. The first one is from the most-used embryology textbook:
“Although life is a continuous process, fertilization (which, incidentally, is not a ‘moment’) is a critical landmark because, under ordinary circumstances, a new genetically distinct human organism is formed when the chromosomes of the male and female pronuclei blend in the oocyte.” 
“A zygote is the beginning of a new human being (i.e. an embryo).” 
To top it all off, past president of Planned Parenthood, Alan Guttmacher, wrote the following concerning the scientific fact of the unborn human being in 1933, a full forty years before Roe v. Wade:
“This all seems so simple and evident that it is difficult to picture a time when it wasn’t part of the common knowledge.” 
Pro-choice philosophers accept this scientific fact, as well:
“It is possible give ‘human being’ a precise meaning. We can use it as equivalent to ‘member of the species Homo sapiens’. Whether a being is a member of a given species is something that can be determined scientifically, by an examination of the nature of the chromosomes in the cells of living organisms. In this sense there is no doubt that from the first moments of its existence an embryo conceived from human sperm and eggs is a human being.” 
“Prior to conception there is only a sperm and an ovum. As these are both necessary for bringing somebody into existence, but because they are distinct entities prior to conception, they cannot be identical with the being that will be brought into existence. Two cannot be identical with one. Thus we cannot speak of a new organism as having come into existence prior to conception. Put another way, each one of us was once a zygote, but none of us was ever a sperm or an (unfertilized) ovum.” 
Conclusion One: Therefore, abortion is prima facie immoral.
If it is prima facie immoral to kill a human being, and abortion kills a human being, then it naturally follows that abortion is prima facie immoral. The burden of proof is on the pro-choice advocate to show that abortion is morally justified.
Abortions are, in fact, justified to save the life of the pregnant woman if her life is in immediate jeopardy and the child is not yet viable. If the child is viable, the child must be delivered (not only to save the child, but it is safer and faster for the mother to deliver the child). If the child is not yet viable, then the doctor must do the greatest good he can. If only one can be saved, the doctor must save the one with the greatest chance of survival (the mother).
In all other cases, abortion is simply not morally justified.
Premise Three: The unjustified killing of human beings should be illegal.
By unjustified, I simply mean killing a human being without moral justification. It is wrong to go and kill a human being without just cause. And humans are uniquely valuable (as I have shown), so valuable that society does, in fact, make their killing illegal. It doesn’t just take moral justification to kill a human being, it takes strong moral justification to kill one. Killing a human being without just cause should be illegal, and is made illegal by civilized societies. If the unborn are human beings, which is strongly supported by science, then killing them should be illegal, as well.
Conclusion Two: Abortion should be illegal.
My second conclusion naturally follows. If we make the unjustified killing of human beings illegal, which the unborn certainly are, then killing them should also be illegal.
Now that my argument has been supported, how does one respond?
There are three ways you could respond to an argument:
1) Search for logical fallacies. I have shown several logical fallacies and some of the arguments made by pro-life and pro-choice people in my previous article. My argument here does not contain any logical fallacies. I have supported my premises with arguments, and the conclusions naturally follow.
2) Rebut the premises. If there are no logical fallacies, then what remains is to argue against the premises. You could tackle premise two, but you’re arguing against science if you try to do so. Embryologists (the experts on human embryology) consistently agree that human beings are human beings from fertilization. The only people I’ve ever heard contest this are lay pro-choice people who are so desperate to prove that abortion is moral that they are willing to argue against science. But I’ve always found it bizarre that people will accept science’s word when it comes to evolution, but suddenly science can’t be trusted when it comes to determining human life.
However, the best pro-choice arguments attack premise one. Pro-choice people agree that it is prima facie immoral to kill a human being, but they believe that abortion is justified killing. If you can show that abortion is justified killing, my argument fails to show that abortion is immoral. This is usually done by arguing for bodily rights (the best defenders of this being Judith Jarvis Thomson and David Boonin). But even then, bodily rights arguments are not sufficient to show that abortion is moral or should be legal. 
3) Concede the argument. If there are no logical fallacies and there are no arguments strong enough to refute it, then the only thing left is to change your views. I fully admit I could be mistaken in my views. I don’t believe that I am, and I’m open to discussion about my argument. However, I have read the best arguments on both sides of the equation (from pro-life, as well as pro-choice, philosophers). The best arguments, with the greatest explanatory power, lie on the pro-life side.
In my next and last article in the series, I will discuss bad arguments from both sides of the abortion fence.
 Ronan O’Rahilly and Fabiola Muller, Human Embryology and Teratology, 3rd ed., New York: Wiley-Liss, 2001, p. 8.
 Keith L. Moore, The Developing Human: Clinically Oriented Embryology, 7th ed., Philadelphia, PA: Saunders, 2003, p. 2.
 Alan Guttmacher, Life in the Making: The Story of Human Procreation, New York: Viking Press, 1933, p. 3.
 Peter Singer, Practical Ethics, 2nd ed. (Cambridge: Cambridge University Press, 1993), pp. 85-86.
 David Benatar, Better Never to Have Been: The Harm of Coming into Existence, (Oxford University Press: Oxford, New York, 2006), p. 134