I have given you my own basic argument for the pro-life position. But now I’d like to examine two more arguments, what I consider to be the two strongest arguments for the pro-life position. The first is by a Catholic philosopher, whose argument is grounded in intuitions that most people agree with. The next argument will be by an atheist (I’ll leave who it is a mystery, but those who are well-read in the abortion issue will likely know who I’m talking about).
In his book Defending Life: A Moral and Legal Case Against Abortion Choice, philosopher Frank Beckwith makes an argument that he has deemed the Substance View. His argument is as follows:
1. The unborn entity, from fertilization , is a full-fledged member of the human community.
2. It is prima facie morally wrong to kill any member of that community.
3. Every successful abortion kills an unborn entity, a full-fledged member of the human community.
4. Therefore, every successful abortion is prima facie morally wrong. 
(Frank Beckwith, Defending Life: A Moral and Legal Case Against Abortion Choice, Cambridge: University Press, Cambridge, New York, 2007, p. xii.)
This is an argument I use in my discussions on abortion, as well as my previous argument. In fact, this is the argument that I have adapted my own argument from. Beckwith’s position here relies on the continuum of human essence. In other words, it is morally wrong to kill you now. You are the same entity now as you were in the womb. So it was morally wrong to kill you then, as well.
As I mentioned, Beckwith is a Catholic. So he does believe in the concept of a soul. However, even if you don’t believe in a soul you can at least see that there is a continuity of human essence. You have developed more as a human being, you have developed more skills and basic human abilities (such as the ability to walk, talk, etc.). You have grown taller (perhaps wider). You may have dyed your hair, gotten a piercing or tattoo, broken a bone, gone through puberty, and/or have gone through any number of other changes since you were conceived. But who you are, your substance, hasn’t changed. A substance is simply an entity that maintains its identity through change. The human being is a particular type of substance, a rational, moral agent, and remains one until he/she dies.
Some might argue that we’re not the same entity as we were in the womb because we had no experiences and we have no memory of being in the womb. This argument is mistaken because first, you have no memory of being a newborn or a toddler, either. Yet you are undoubtedly the same entity then as you are now. Second, if your mom had miscarried or aborted you, you would not be here today. The same as if you had died when you were a toddler or newborn. Third, the unborn do have experiences. Even if you don’t consciously remember your experiences, you still have them. The unborn are pushed down the fallopian tubes toward the uterus by tiny hairs called cilia. My friend and pro-life advocate Josh Brahm calls this your first “waterslide ride.” The unborn implants in the uterus. These are all experiences that the unborn have, even if they can’t remember them.
Another objection I occasionally encounter is that our cells die and replace themselves every seven years, so we are not the same person we were seven years ago. However, this doesn’t follow. You are still the same “you” you were seven years ago (and seventeen years ago, and when you were born, and when you were conceived). We all experience changes, but we don’t “cease to exist” every seven years just because our cells replace themselves. The experiences we had before our cells replicated were still had by the same “substance.” Provided we are old enough, we can remember back to times in our lives from before we had our current cells. We may go through changes, but the essence that makes us “us” doesn’t change.
Beckwith has devoted an entire book to defending his argument (and it’s one of the strongest books defending the pro-life position), so obviously it would be impossible to give a full treatment and defense of the argument here. This is the backbone of the argument, and I really think it’s the strongest argument we have (though there are, of course, those who disagree). Beckwith’s argument is made from a non-religious standpoint. Even if you don’t believe in the same theological concepts that he ascribes to, you can’t dismiss the argument simply because it’s made by a Catholic. As he says in his book, if Beckwith’s argument is sound, an atheist, agnostic, or humanist is intellectually obligated to become pro-life. (Beckwith, Defending Life, p. xiv.)
The preceding article also appeared on the Secular Pro-Life blog.
 Beckwith argues from the "moment of conception." I have changed this to fertilization. Conception is not actually a "moment," and the process of bringing a human into existence occurs sometime during the fertilization process, even though the exact point has not yet been agreed upon (Beckwith also mentions this later in his book). So I have substituted fertilization because I feel it's slightly more accurate.
 It should be noted that if the Substance View succeeds, then even unsuccessful abortions are immoral since it is wrong to even attempt to take someone's life, even if the actual outcome was less than was intended (or if no harm actually arose).
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