"The closest event we can find to a thunderclap marking the entry of a soul into the world is the moment of conception. At that instant a new human genome is determined, and we have an entity destined to develop into a unique individual.
The Catholic Church and certain other Christian denominations designate conception as the moment of ensoulment and the beginning of life (which, of course, makes abortion a form of murder). But just as a microscope reveals that a straight edge is really ragged, research on human reproduction shows that the “moment of conception” is not a moment at all.
Sometimes several sperm penetrate the outer membrane of the egg, and it takes time for the egg to eject the extra chromosomes. What and where is the soul during this interval? Even when a single sperm enters, its genes remain separate from those of the egg for a day or more, and it takes yet another day or so for the newly merged genome to control the cell. So the “moment” of conception is in fact a span of twenty-four to forty-eight hours.
Nor is the conceptus destined to become a baby. Between two-thirds and three-quarters of them never implant in the uterus and are spontaneously aborted, some because they are genetically defective, others for no discernible reason. Still, one might say that at whatever point during this interlude the new genome is formed, the specification of a unique new person has come into existence. The soul, by this reasoning, may be identified with the genome. But during the next few days, as the embryo's cells begin to divide, they can split into several embryos, which develop into identical twins, triplets, and so on.
Do identical twins share a soul? Did the Dionne quintuplets make do with one fifth of a soul each? If not, where did the four extra souls come from? Indeed, every cell in the growing embryo is capable, with the right manipulations, of becoming a new embryo that can grow into a child. Does a multicell embryo consist of one soul per cell, and if so, where do the other souls go when the cells lose that ability? And not only can one embryo become two people, but two embryos can become one person.
Occasionally two fertilized eggs, which ordinarily would go on to become fraternal twins, merge into a single embryo that develops into a person who is a genetic chimera: some of her cells have one genome, others have another genome. Does her body house two souls? For that matter, if human cloning ever became possible (and there appears to be no technical obstacle), every cell in a person's body would have the special ability that is supposedly unique to a conceptus, namely developing into a human being.
True, the genes in a cheek cell can become a person only with unnatural intervention, but that is just as true for an egg that is fertilized in vitro. Yet no one would deny that children conceived by IVF have souls.
The idea that ensoulment takes place at conception is not only hard to reconcile with biology but does not have the moral superiority credited to it. It implies that we should prosecute users of intrauterine contraceptive devices and the “morning-after pill” for murder, because they prevent the conceptus from implanting.
It implies that we should divert medical research from curing cancer and heart disease to preventing the spontaneous miscarriages of vast numbers of microscopic conceptuses. It impels us to find surrogate mothers for the large number of embryos left over from IVF that are currently sitting in fertility clinic freezers.
It would outlaw research on conception and early embryonic development that promises to reduce infertility, birth defects, and pediatric cancer, and research on stem cells that could lead to treatments for Alzheimer's disease, Parkinson's disease, diabetes, and spinal-cord injuries.
And it flouts the key moral intuition that other people are worthy of moral consideration because of their feelings — their ability to love, think, plan, enjoy, and suffer — all of which depend on a functioning nervous system. The belief that bodies are invested with souls is not just a product of religious doctrine but embedded in people's psychology and likely to emerge whenever they have not digested the findings of biology.
The discovery that what we call “the person” emerges piecemeal from a gradually developing brain forces us to reframe problems in bioethics. It would have been convenient if biologists had discovered a point at which the brain is fully assembled and is plugged in and turned on for the first time, but that is not how brains work.
The nervous system emerges in the embryo as a simple tube and differentiates into a brain and spinal cord. The brain begins to function in the fetus, but it continues to wire itself well into childhood and even adolescence. The demand by both religious and secular ethicists that we identify the “criteria for personhood” assumes that a dividing line in brain development can be found. But any claim that such a line has been sighted leads to moral absurdities.
If we set the boundary for personhood at birth, we should be prepared to allow an abortion minutes before birth, despite the lack of any significant difference between a late-term fetus and a neonate. It seems more reasonable to draw the line at viability. But viability is a continuum that depends on the state of current biomedical technology and on the risks of impairment that parents are willing to tolerate in their child. And it invites the obvious rejoinder: if it is all right to abort a twenty-four-week fetus, then why not the barely distinguishable fetus of twenty-four weeks plus one day?
And if that is permissible, why not a fetus of twenty-four weeks plus two days, or three days, and so on until birth? On the other hand, if it is impermissible to abort a fetus the day before its birth, then what about two days before, and three days, and so on, all the way back to conception?
We face the same problem in reverse when considering euthanasia and living wills at the end of life. Most people do not depart this world in a puff of smoke but suffer a gradual and uneven breakdown of the various parts of the brain and body. Many kinds and degrees of existence lie between the living and the dead, and that will become even more true as medical technology improves.
We face the problem again in grappling with demands for animal rights. Activists who grant the right to life to any sentient being must conclude that a hamburger eater is a party to murder and that a rodent exterminator is a perpetrator of mass murder. They must outlaw medical research that would sacrifice a few mice but save a million children from painful deaths (since no one would agree to drafting a few human beings for such experiments, and on this view mice have the rights we ordinarily grant to people). On the other hand, an opponent of animal rights who maintains that personhood comes from being a member of Homo sapiens is just a species bigot, no more
thoughtful than the race bigots who value the lives of whites more than blacks.
After all, other mammals fight to stay alive, appear to experience pleasure, and undergo pain, fear, and stress when their well-being is compromised. The great apes also share our higher pleasures of curiosity and love of kin, and our deeper aches of boredom, loneliness, and grief. Why should those interests be respected for our species but not for others?
Some moral philosophers try to thread a boundary across this treacherous landscape by equating personhood with cognitive traits that humans happen to possess. These include an ability to reflect upon oneself as a continuous locus of consciousness, to form and savor plans for the future, to dread death, and to express a choice not to die.
At first glance the boundary is appealing because it puts humans on one side and animals and conceptuses on the other. But it also implies that nothing is wrong with killing unwanted newborns, the senile, and the mentally handicapped, who lack the qualifying traits. Almost no one is willing to accept a criterion with those implications.
There is no solution to these dilemmas, because they arise out of a fundamental incommensurability: between our intuitive psychology, with its all-or-none concept of a person or soul, and the brute facts of biology, which tell us that the human brain evolved gradually, develops gradually, and can die gradually. And that means that moral conundrums such as abortion, euthanasia, and animal rights will never be resolved in a decisive and intuitively satisfying way.
This does not mean that no policy is defensible and that the whole matter should be left to personal taste, political power, or religious dogma. As the bioethicist Ronald Green has pointed out, it just means we have to reconceptualize the problem: from finding a boundary in nature to choosing a boundary that best trades off the conflicting goods and evils for each policy dilemma.
We should make decisions in each case that can be practically implemented, that maximize happiness, and that minimize current and future suffering. Many of our current policies
are already compromises of this sort: research on animals is permitted but regulated; a late-term fetus is not awarded full legal status as a person but may not be aborted unless it is necessary to protect the mother's life or health.
Green notes that the shift from finding boundaries to choosing boundaries is a conceptual revolution of Copernican proportions. But the old conceptualization, which amounts to trying to pinpoint when the ghost enters the machine, is scientifically untenable and has no business guiding policy in the twenty-first century. The traditional argument against pragmatic, case-by-case decisions is that they lead to slippery slopes. If we allow abortion, we will soon allow infanticide; if we permit research on stem cells, we will bring on a Brave New World of government-engineered humans. But here, I think, the nature of human cognition can get us out of the dilemma rather than pushing us into one. A slippery slope assumes that conceptual categories must have crisp boundaries that allow in-or-out decisions, or else anything goes. But that is not how human concepts work.
As we have seen, many everyday concepts have fuzzy boundaries, and the mind distinguishes between a fuzzy boundary and no boundary at all. 'Adult' and 'child' are fuzzy categories, which is why we could raise the drinking age to twenty-one or lower the voting age to eighteen. But that did not put us on a slippery slope in which we eventually raised the drinking age to fifty or lowered the voting age to five. Those policies really would violate our concepts of 'child' and 'adult,' fuzzy though their boundaries may be. In the same way, we can bring our concepts of life and
mind into register with biological reality without necessarily slipping down a slope."
-Steven Pinker, The Blank Slate