On February 8, 2002, Jean Porter--who is the John A. O'Brien Professor of Theology at the University of Notre Dame, specializing in moral theology/Christian ethics--published an article in Commonweal:  Is the Embryo a Person?  Arguing with the Catholic Traditions.  Click here to read the whole article.  Here is a lengthy excerpt:

What can we say to convince men and women of good will who do not share our theological convictions or our allegiance to a teaching church that early-stage embryos have exactly the same moral status as we and they do? It will not serve us to fall back at this point on blanket denunciations such as "the culture of death." Naturally, these tend to be conversation stoppers. What is worse, they keep us from considering the possibility that others may not be convinced by what we are saying because what we are saying is--not convincing.
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According to Catholic teaching, from conception, the embryo has the same status as any other living human being, and for this reason, the intentional destruction of the embryo is tantamount to murder. That, at least, is the official teaching of the Catholic Church as it has been represented in the press. The official teaching is qualified in one important respect, and I will consider this qualification in a moment. But first we need to examine what might be called the simplified public version of the teaching on the status of the early embryo.

Very often, the claim that the early-stage embryo is a person just like any other is put forward as if it were obvious. When pressed to defend this view, its proponents argue that the early-stage embryo is a living human organism, genetically distinct from both its parents and already on a developmental continuum that will lead eventually to the birth of a child. From this standpoint, it is obvious that this individual human organism must also be a human person in the fullest sense. As Donum vitae asks rhetorically, "How could a human individual not be a human person?"

Yet this is precisely what is not obvious to many of our fellow citizens. What is more, until relatively recently it would not have been obvious to most Catholic theologians, either. The view that even early abortion is equivalent to murder did not begin to dominate official Catholic teachings until the nineteenth century, although it had been proposed earlier. Before that point, the majority view in the Western church, as reflected in canon law as well as theological opinion, drew a distinction between early- and late-stage abortions. Certainly, an early-stage abortion was considered to be a grave sin, but it was not regarded as equivalent to murder. This distinction, in turn, rests on the view, defended by Aquinas among many others, that the developing fetus does not receive a rational soul, and therefore does not attain full human status, until after a certain point in the process of development-a view sometimes described as "delayed hominization." These theologians were well aware that the early-stage embryo was an organism distinct from the body of its mother, but they denied precisely what we are now claiming to be not only true, but obviously true-namely, that this human organism is a human person in the full sense.

if we are going to defend immediate hominization, it is not enough simply to point to facts about the early-stage embryo-its genetic uniqueness, its capacity for further growth and development, and the like. We need to offer a systematic argument for why these facts support the fully personal status of very early-stage embryos, an argument that will convince our fellow citizens, most of whom know the facts as well as we do. This we have so far failed to do.

That fact brings me to the qualification in the official teaching on the status of the early embryo noted above. When we turn to recent magisterial statements, we find that they do acknowledge the difficulty of this question and the diversity of views that it has engendered. But these debates finally do not matter, because even if we cannot say for sure when the embryo attains fully personal status, we are sure enough to act as if it does: "What is at stake is so important that, from the standpoint of moral obligation," John Paul II writes in Evangelium vitae, "the mere probability that a human person is involved would suffice to justify an absolutely clear prohibition of any intervention aimed at killing a human embryo." The encyclical speaks of a probability, but some theologians and Catholic activists go even further, arguing that the possibility that the embryo is a person is enough to justify treating it as if it were. In other words, we are faced here with a situation in which a fully personal human life may be present, and therefore we are morally obliged to resolve our doubts on the side of protecting life.

But these are weak arguments, at least insofar as they are offered as a basis for public policy. Have we established even a probability that the early-stage embryo has fully personal status? This claim stands or falls on an assessment of the strength of our arguments. So far, it does not seem that we have convinced our fellow citizens that these arguments are sound enough to justify even a probable conclusion. As for the contention that the sheer possibility that the early-stage embryo is a fully personal entity requires us to act as if it were, this claim will rightly not carry much weight in the public square.

What this latter claim amounts to is this: Because there are philosophical arguments to the effect that the early-stage embryo is fully personal, and these arguments convince some persons, therefore we should all act as if we were convinced by those arguments. But no society can afford to determine difficult questions of public policy on such a basis. There are serious philosophical arguments that nonhuman animals have the same status as human persons, and these arguments convince many men and women of good faith. Does that mean that the rest of us are morally obliged to act as if we were convinced by those arguments, because nonhuman animals may have the same moral status as the rest of us? Should we outlaw animal experimentation and slaughterhouses on this basis? I think most of us would demand to be convinced by the relevant arguments, before we think about enacting them into law. We should not be surprised when our fellow citizens hold us to the same standards of persuasiveness with regard to the status of the early embryo.

No doubt it will have occurred to many of my readers that if our current position on the status of the early embryo is not convincing to our fellow citizens, we will surely not get much further with Aquinas’s abstruse metaphysics. Actually, I am not so sure about that. I myself find Aquinas’s view to be convincing, and since I believe that this view is intrinsically credible, I am inclined to think that at least some of our fellow citizens would believe it if it were fairly presented. We have been in the habit of dismissing this view for so long that we have not even tried to understand it ourselves, much less to render it intelligible to others.

Nonetheless, it is probably the case that Aquinas’s views on ensoulment, which after all do presuppose a fairly specific set of philosophical and theological assumptions, will never carry much general conviction. And many Catholics will believe that whatever the merits of this view, any effort to develop it in a direction that would allow for the destruction of early-stage embryos has been definitively ruled out by the magisterium. (I do not agree with this position, although I respect it. For the record, and since I will be accused of cowardice unless I declare my own position, I do not believe that there are any conclusive moral grounds ruling out embryonic stem-cell research. Hence, I do support such research, under some conditions. I reserve the right to change my mind. And I don’t know what Aquinas himself would have said on the subject.)

My point is not that we should return to the earlier position on ensoulment and the status of the early embryo. What I do want to suggest, however, is that if we are to develop adequate and convincing arguments on this difficult issue, we need to engage the arguments of our forerunners in a serious way-especially those arguments that we find most challenging to our own views. In this case, it would be especially advisable for us to do so, because the arguments of our forerunners challenge today’s official Catholic view just at the point at which most of our contemporaries would also challenge it-that is, they call into question the claim that the early-stage embryo, as a human organism, must necessarily be a human person in the full sense. If we cannot answer our own forerunners’ objections to this claim, we can scarcely hope to answer the objections of our contemporaries.

Catholics are defined by our past in a way that many of our fellow citizens are not. Not only does the institutional church give a high positive value to continuity and tradition, most individual Catholics do so as well. Yet ironically, when we attempt to bring our insights to bear on public debates, we sometimes argue in ways that undercut the very continuity with tradition that has historically been our strength. I do not want to suggest that we should simply retreat into an uncritical affirmation of the views of the past. But I do want to say that when we find ourselves at variance with our inherited moral tradition, we should face that fact squarely and attempt to develop our own views in conversation with our forerunners, taking their views seriously even when these are not our own. Only in this way can we maintain a genuine, and not merely a verbal respect for our inherited tradition-and only in this way can we develop arguments that will be convincing to our fellow citizens today.