John T. Noonan, a Catholic jurist whose work on abortion regularly features in ethics textbooks, contends that the traditional definition of a human being remains rationally superior to its competitors. A human being, Noonan insists, is anyone who has been conceived by human parents.

The most common rival to conception is that of viability: the fetus becomes a human being once it is viable, i.e. capable of surviving outside of the mother’s womb.

Noonan notes what he takes to be three key problems with this humanity-defining criterion.

First, the notion of viability is the hard and fast standard that it is thought to be. It possesses “considerable elasticity.”

Second, this last point is proven by the fact that, given the enormous advances in technology, the fetus, courtesy of artificial incubation, could be “viable” at almost any time.

Third, viability, in the sense of self-sufficiency, occurs neither with birth nor even well after birth. Infants, toddlers, and small children remain utterly dependent upon the care of others.

That Noonan is correct about the inadequacy of viability as a criterion for humanity is true enough. But it should be noted that his last objection against the viability thesis commits the fallacy of equivocation: When proponents of so-called “abortion rights” appeal to the viability of the fetus, Noonan must know, they allude to the self-evident fact that until such time, the fetus is uniquely dependent upon its mother. Thus, they reason, it is up to the mother to determine whether she will continue to care for the fetus until such time that others could assume responsibility. Yet once others can assume responsibility, then the fetus is no longer uniquely dependent upon its mother.

Of course, none of this should be interpreted as a defense of the claim that the unique dependence of a fetus upon its mother endows the latter with a “right” to abort. But there can be no progress on this issue if clarity is lost to us.

Noonan also considers the standard of experience: “A being who has had experience, has lived and suffered, who possesses memories, is more human than one who has not.”

The problem with this account is twofold.

First, from the earliest stages of pregnancy, the pre-natal entity—first the zygote, then the embryo—is experiencing its surroundings. So, by the gauge of “experience,” the zygote is a human being.

Second, if what is meant is that the more experience a being has, the more human it is, than what this implies is that whole classes of beings otherwise considered human—children, the young, etc.—are either not human or minimally human.  Noonan also cites the admittedly rare case when aphasia has deleted its victim’s memories.  If experience is the test of humanity, then must we conclude that along with a person’s memories, aphasia also eliminates his or her humanity?

Here, it would be helpful to know how those who appeal to experience in determining humanity employ this concept. “Experience” is a concept that admits of a multitude of conceptions, to borrow Ronald Dworkin’s terminology. It is an empty concept until it is given content.

Typically, “experience” is invoked when it is meant to refer to sentience, the capacity for pleasure and, more importantly, pain. The zygote, it is obvious, is not sentient.  Neither, for that matter, is the fetus prior to a certain juncture.  Still, it is anything but a foregone conclusion that sentience bestows humanity—animals of various sorts are sentient too—and it certainly isn’t axiomatic that sentience gives rise to rights.

Noonan also deems as wanting the sentiments of adults and the sensations of parents as criteria for humanity.  They share the same fundamental problem: both imply that the humanity of a being—in this case, the fetus—is a dispensation bestowed by others. Noonan, however, expects for us to recognize that this just can’t be correct. Here he appeals to the historical record while reminding us that these are precisely the criteria relied upon by those  members of religious, racial, and other groups who, because of their inability to empathize with the members of other such groups, have dehumanized  the latter.

Finally, Noonan contends that social recognition is equally insufficient to establish humanity—and for basically the same reason that these last two criteria fail.  If “society”—meaning the majority—constitutes the ultimate arbiter of who is human, then those of its members who at any given moment are unpopular or held to be undesirable could find themselves divested of their humanity.  Again, he revisits history to remind us of the great evils to which this position has lead.

In the last analysis, Noonan concludes, there is only one non-arbitrary line by which to demarcate the human from the non-human.  And this line is conception.

It is at conception that a self-evolving entity with a unique genetic code—a human genetic code—comes into being.  It is at this moment that a human being emerges.

Abortion, then, is nothing more or less than the killing of a human being. As such, it is almost always wrong.

Noonan, I believe, has indeed succeeded in laying waste to the five humanity-defining criteria on which he sets his sights. That being said, his argument is vulnerable to two criticisms.

The first is that in exposing weaknesses in each of the criteria, Noonan does not necessarily reveal their arbitrariness: the criteria can be inadequate without being arbitrary.  Just because there is some measure of imprecision in a standard does not mean that the standard is without its effectiveness in doing the work assigned to it.

To assume otherwise, it seems, is to presuppose that, at least when it comes to the question of defining a human being, any list of criteria that isn’t exhaustive is capricious and, thus, worthless. But perhaps a search for something like necessary and sufficient conditions with respect to this issue is an exercise in futility.  

The second fault to be found in Noonan’s reasoning is that he uses the same idiom as his opponents: Abortion, in these terms, pertains to the relationship between one human being—“the fetus”—and another—a pregnant woman. He further ensconces this scheme when he expressly says of abortion that it violates the Christian injunction to “love one’s neighbor as oneself.”

The problem, however, is that the unborn is not just another neighbor of its mother. The relationship between a woman and her child, especially when that child is in her womb, utterly dependent upon its mother and her alone for its sustenance and protection, is radically unlike any other.

Any discussion of abortion that fails to take this fact into account falls short.