Several ethicists, such as Michael Tooley, Mary Anne Warren, James Rachels, and Virginia Ramey Mollenkott, have put forth criteria that a being must fulfill in order to be considered fully human. For some these criteria apply to any entity, whether before or after birth. In fact, according to Tooley, birth has no bearing on the moral status of the newborn.
Those who defend criteria for full humanness make a distinction between “being a human” and “being a person.” They argue that although the unborn are part of the species Homo sapiens, and in that sense are human, they are not truly persons since they fail to fulfill a particular set of personhood criteria.
Although the defenders of personhood criteria do not agree on everything, their underlying philosophical assumptions are similar enough that it is safe to say that if I can show that these assumptions are significantly flawed then no personhood criteria theory can succeed in supporting the abortion-rights position.
Since Mollenkott's view is the most clear and succinct example, I will use her article as my point of departure to critique the personhood criteria position. Although much of my critique of this view can be found in my criticisms of the other decisive moment and gradualist theories, its underlying philosophical assumptions, which are oftentimes not addressed by the proponents of this view, are deserving of a separate critique.
In order to fully grasp Mollenkott's position, let me quote her at length:
Mollenkott's argument can be put in the following argument-outline:
Others, such as Tooley and Warren, give more elaborate criteria of human personhood. For instance, Tooley claims that a being “cannot have a right to continued existence unless he possesses the concept of a subject of experiences, the concept of a temporal order, and the concept of identity of things over time.” It follows that a nonself-conscious being with no desire for its own continued existence has no right to life.
Hence, the unborn do not have a right to life. In any event, the philosophical assumption behind both Mollenkott's and Tooley's arguments, as well as the arguments of others such as Warren and Rachels, is that only an entity that functions in a certain way (e.g., in the case of Tooley, “is capable of desiring that p be the case”) is a person with a full right to life (i.e., fully human).
I maintain that this position has several flaws.
First, it does not seem to follow from the intermediate conclusion (that an unborn human is not a person) that abortion is always morally justified. Jane English has pointed out that “non-persons do get some consideration in our moral code, though of course they do not have the same rights as persons have (and in general they do not have moral responsibilities), and though their interests may be overridden by the interests of persons. Still, we cannot just treat them in any way at all.” 
English goes on to write that we consider it morally wrong to torture beings that are nonpersons, such as dogs or birds, although we do not say these beings have the same rights as persons. And though she considers it problematic as to how we are to decide what one may or may not do to nonpersons, she nevertheless draws the conclusion that “if our moral rules allowed people to treat some person-like non-persons in ways we do not want people to be treated, this would undermine the system of sympathies and attitudes that makes the ethical system work.” 
Second, one can question why one must accept a functional definition of personhood to exclude the unborn. It is not obvious that functional definitions always succeed. For example, when the Boston Celtics' Larry Bird is kissing his wife, does he cease to be a basketball player because he is not functioning as one? Of course not. He does not become a basketball player when he functions as a basketball player, but rather, he functions as a basketball player because he is a basketball player.
Similarly, when a person is asleep, unconscious, or temporarily comatose, or a newborn, he (or she) is not functioning as a person as defined in premise 2. Nevertheless, no reasonable person would say that this individual is not a person while in this state. 
Therefore, since a person functions as a person because he is a person and is not a person because he functions as a person, defining personhood strictly in terms of function is inadequate.
Of course, the abortion-rights advocate may want to argue, as was argued in the case of the sentience criterion, that the analogy between sleeping/unconscious/comatose persons and the unborn breaks down because the former at one time in their existence functioned as persons while the latter, the unborn, did not. Although this point is worth noting, the abortion-rights advocate fails to grasp the significant flaw in defining personhood strictly in terms of function.
As I pointed out in my criticism of the sentience criterion, to claim that a person can be functional, become nonfunctional, and then return to a state of function is to assume that there is some underlying personal unity to this individual that allows us to say that the person who has returned to functional capacity is the same person who was functional prior to being in a nonfunctional state. But this would mean that human function is a sufficient but not a necessary condition for personhood.
Consequently, it does not make sense to say that a person comes into existence when human function arises. Rather, it does make sense to say that a fully human entity is a person who has the natural inherent capacity to give rise to human functions. And since an unborn entity typically has this natural inherent capacity, (he or) she is a person.
As John Jefferson Davis writes, “Our ability to have conscious experiences and recollections arises out of our personhood; the basic metaphysical reality of personhood precedes the unfolding of the conscious abilities inherent in it.” Therefore, an ordinary unborn human entity is a person, and hence, fully human.
In other words, because the unborn human is a person with a certain natural inherent capacity (i.e., her essence), she will function as a person in the near future, just as the reversibly comatose and the temporarily unconscious will likewise do because of their natural inherent capacity. The unborn are not potential persons but persons with much potential.
Along the same lines, Ray has made the observation that the view of human person as a natural “kind” which provides a ground for certain functions, rather than as an emergence of certain functions, is more consistent with our general moral intuitions. For “the recognition of the rights of the young is less dependent on their actual, current capacities than on their species and potential [i.e., their natural inherent capacity].”
For example, no one doubts that day-old human children have fewer actual capacities than day-old calves. Human infants, in terms of environmental awareness, mobility, etc., are rather unimpressive in comparison to the calves, especially if one calculates their ages from conception. But this comparison does not persuade us to believe that the calves have greater intrinsic worth and an inherent right to life. For if human infants were sold to butchers (let us suppose for the high market value of their body parts) in the same way that farmers sell calves to humane butchers, we would find such a practice deeply disturbing.
Yet if intrinsic worth is really contingent upon current capacities rather than natural inherent capacity, we should have no problem with the selling of human infants to butchers. But Ray points out why we do find such a practice morally repugnant:
Author: Francis J. Beckwith, adapted from a series in Christian Research Journal, Spring 1991. Provided with permission by Summit Ministries and the author.
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