Nat Hentoff has invited readers to respond to an argument made by the editors of “Commonweal”: “Those who hold that abortion is immoral believe that the biological dividing lines of birth and viability should no more determine whether a developing member of the species be denied or accorded essential rights than should the biological dividing lines of sex or race or disability or old age.”

To begin with, it’s worth trying to specify why killing is wrong at all. What does it mean to say that life is sacred? Few people advocate compulsory vegetarianism; for most of us, only human life is sacred. What is the difference between human and nonhuman life? Humans have ideas, passions, desires, hopes, memories, expectations, intentions—that is, feelings and mental experiences of a complexity and intensity apparently not found in other species. These feelings and thoughts are inestimably precious; they are what make life worth living; having them is arguably what “being human” means. When a human creature stops having them— through brain death or some other irreparable calamity—we don’t, in practice, consider him or her fully human any longer; and whether we deny or accord such creatures any further life depends not on their wishes—they have none—but on their families’ and care takers’ wishes.

A fetus does not have ideas, passions, desires, hopes, memories, expectations, or intentions. As far as I know, no one claims otherwise, even those who are certain that a fetus is a person (whatever they may mean). Until late in pregnancy, fetuses have rudimentary nervous systems; they can feel pain, but probably less intense and exquisite pains than many adult mammals whom we annually slaughter by the hundreds of millions for reasons far less urgent than those of women seeking abortions. Why, then, might anyone believe that life of a fetus is sacred?

I can imagine three reasons. First, its parents cherish hopes for it and would suffer greatly if it died. But these fetuses—they are the large majority—are not in any danger; on the contrary, they are cared for with all the resources the free market allows them. Second, it receives from God an immaterial, immortal soul at the moment of conception. Hentoff and the editors of “Commonweal” should not underestimate how many people believe that this is why an unwanted fetus has an “essential right” to life, and thus to what extent the outlawing of abortion would be religiously motivated. Third, a fetus is a potential person: if it survives, it will eventually have ideas, passions, desires, etc. Everyone can agree on this, but what follows from it? As far as I can see, nothing. Abortion is not the only practice that limits the number of actual persons in the world. If morality requires that the number of potential persons who become actual ones be maximized, then not only abortion but also contraception and celibacy must be prohibited. It is arbitrary to restrict potential personhood to fetuses; every unfertilized ovum—for that matter, ever sexual stirring—is a potential person. And the argument from potentiality gives rise to other absurdities as well. For example, “Commonweal” refers to the fetus as “a developing member of the species”: this phrase elides the difference between a child, whom everyone agrees is a person, and a zygote, which, by any nontheological definition, clearly is not.

Of course it’s not merely a question of errors in logic. The main difference between an ovum and a zygote is a political one. There are infinite number of potential persons awaiting actualization. Why not compel every woman to have an unwanted child, rather than only t hose women who become accidentally pregnant? The answer is obvious: what’s wanted is not the actualization of potential persons, but to make women fearful (the cant word is “responsible”) about having sex.

Unlike fetuses, pregnant women are actual persons. The suffering involved in bringing an unwanted pregnancy to term and then giving up the baby for adoption (or raising it reluctantly) is acute, long lasting, and—unlike the pain registered by the fetus’s comparatively primitive nervous system—not at all hypothetical. Hentoff has been scathing about the euphemisms employed by his antagonists in the “quality of life” camp. But his characterization of noneconomic motives for abortion as mere matters of “convenience” is as misleading and uncharitable as anything he’s cited by the other side.

A potential person has rights only when an actual person (i.e., the one who has crated and must sustain its potential) wants it, because only an actual person can want. Herein lies the answer to Hentoff’s chief worry: that “ ‘convenience’ abortions are affecting societal attitudes toward erasing others of the inconvenient besides the unborn.” If an “inconvenient” is an actual person—has thoughts, feelings, desires—then it has an “essential right” not merely to life but to the fullness thereof. This criterion excludes unwanted fetuses but includes handicapped children, the aged, and that oppressed majority of adults whose effective erasure from history and debarment from full subjectivity have been passed over without legislative agitation by most of the Catholic hierarchy through many inglorious centuries.