Of all the issues involved in the abortion debate, the question of the personhood of the embryo/fetus is the most decisive. This is because if the embryo, even at the moment of conception, is a person, then any other consideration is secondary. For there are no circumstances in which it would be permissible to kill an innocent person, especially a child—whether those circumstances are relatively trivial (parental inconvenience, say) or very serious (rape, incest, or life of the mother). I don’t think that anyone disagrees that this is the logic of the issue, but whether we accept the conclusion depends on the truth of the proposition that the being in the mother’s womb is a person. I think an irrefutable argument for this can be made from reason alone.
In the case of a later-stage fetus that looks like a human being, and especially if it is twenty weeks or older when it is capable of feeling pain, most of us at the very least would feel deeply uneasy about terminating its life. Person-like appearance and behavior (i.e., reaction to pain, thumb sucking, etc.) account for this moral intuition, I think; but the consistently pro-life view is that the life of a person begins at the moment of conception, so that the zygote—the single-celled organism that comes into being at the moment of conception—is a person in the same sense in which you and I are persons. What is the philosophical, as opposed to the religious, basis for such a claim that, prima facie, sounds so implausible?
When the gamete cells of sperm and egg are joined at the moment of conception, a new thing has come into existence. It is neither sperm nor egg nor separate gamete cells in mere combination but a new kind of being that will persist as a single thing from its beginning to its death. In its inner nature is already contained everything that is essential to its being the kind of thing that it is: intelligence (including conscience), free will, and temperament. Unlike other beings, though, it is not really correct to speak of a human being as being a member of a kind of thing, for part of what makes us human is that each of us is not only a “what” but a “who”—a person. This “who” is already part of the nature that constitutes the being of the human zygote, for the “who” is the most fundamental of the attributes of personhood that will be realized in a concrete way as the life of the human person unfolds in the world. The unfolding of the life that begins as a zygote is the unfolding of the life of both a who (an individual person) and a what (a being of the species “human”). The events in the development of this being are all for the purpose of manifesting this latent though really-existing nature. In other words, what is essential to being a person constitutes the nature of the zygote, for it is a human zygote. But then personhood is present in the zygote as its nature from the first moment of its existence, for how else could its development be oriented toward producing what is uncontroversially a person—say, a two year old? If the unique being of the person were not already present even in the zygote, how could its development aim at it? Put differently, each stage of its development is a stage in the development of a person; therefore, personhood is essential to the being of the zygote (being a zygote is after all only a phase of this “thing” constituted by personhood). But to be a thing whose nature is personhood is to be a person. Hence, the zygote is a person.
We can also see this if we ask what kind of being the zygote “will become”—it will become a human being or a person, of course. But then personhood is always present to it as that at which its development aims. But unless we discern what this means at a deep level, ordinary language will deceive us. For example, to understand what an acorn is, we need to know that it is a kind of nut that contains the seed of an oak. Upon germination, it is no longer simply an acorn but a new thing in development toward becoming an individual tree of a particular sort. It is essentially a thing that is constituted by a sort of pattern, its nature, which determines both what it will become and what it is. Our language lacks a term for the single thing the life of which begins with the germination of the acorn and ends when the tree dies. But from beginning to end it is an individual thing with this essential nature. Someone might argue from the way we conventionally speak of living things that, just as to destroy a germinated acorn is not to destroy an oak, so too it is not the case that to destroy a human zygote is to destroy a human being. But we have seen that this is only to appeal to an accident of ordinary language: we lack a term for the single thing that is the life of the oak from acorn to fully mature tree. In the case of the zygote, we have such a term: person.
Now, I believe that our conscience is grounded in knowledge of reality. At some level, we know that the life of the human person begins at conception. We intuit this when we watch our children grow up and realize that to have killed the being in the mother’s womb that was one of our kids would have been to have destroyed him or her. It would have been to have taken away their future—a future that was already theirs in the womb, that belonged to them by virtue of being the kind of being that they already were. For their future is only a state of their being, just as their present is and their past will be. Past, present and future are all states or stages of a life that begins at conception.
This is why there can be no compromise between the pro-life and pro-choice positions. If the human being is a person even at the zygote stage, then we are never justified in killing him or her. Even in the tough cases (rape, incest, life of the mother), what is also at stake is the life of another person. Thinking of life at its earliest stage, at conception, and picturing it only as a single celled organism, simply does not reveal the reality of the situation. We must think rather of both the distinctly human capacities that exist, latently though really, within the being in the womb and of the life that is in the process of unfolding. To destroy that life even at the beginning is to destroy the life that would have been manifested in the world at every phase: the toddler, the teenager, the parent, the grandmother. It is all one life. And there is still no more fitting name for the principle of that life than the one by which we have always called it: soul. The word “soul” denotes a nature constituted by all those powers, capacities and properties that we take to be distinctly human: intelligence, will, and individual identity as a “who.” Soul is what makes one thing the being that begins as a zygote and ends at natural death. More precisely, it is the principle of life that makes the person’s life at every phase just that: phases of a single life, a human life.