What makes opposition to abortion the issue it is for each of the GOP presidential candidates is the fact that it is the ultimate “wedge issue” -- it is nonnegotiable. The right-to-life people hold that it is as strong a point of religion as any can be. It is religious because the Sixth Commandment (or the Fifth by Catholic count) says, “Thou shalt not kill.” For evangelical Christians, in general, abortion is murder. That is why what others think, what polls say, what looks practical does not matter for them. One must oppose murder, however much rancor or controversy may ensue.

But is abortion murder? Most people think not. Evangelicals may argue that most people in Germany thought it was all right to kill Jews. But the parallel is not valid. Killing Jews was killing persons. It is not demonstrable that killing fetuses is killing persons. Not even evangelicals act as if it were. If so, a woman seeking an abortion would be the most culpable person. She is killing her own child. But the evangelical community does not call for her execution.

About 10% of evangelicals, according to polls, allow for abortion in the case of rape or incest. But the circumstances of conception should not change the nature of the thing conceived. If it is a human person, killing it is punishing it for something it had nothing to do with. We do not kill people because they had a criminal parent.

Nor did the Catholic Church treat abortion as murder in the past. If it had, late-term abortions and miscarriages would have called for treatment of the well-formed fetus as a person, which would require baptism and a Christian burial. That was never the practice. And no wonder. The subject of abortion is not scriptural. For those who make it so central to religion, this seems an odd omission. Abortion is not treated in the Ten Commandments -- or anywhere in Jewish Scripture. It is not treated in the Sermon on the Mount -- or anywhere in the New Testament. It is not treated in the early creeds. It is not treated in the early ecumenical councils.

Lacking scriptural guidance, St. Thomas Aquinas worked from Aristotle’s view of the different kinds of animation -- the nutritive (vegetable) soul, the sensing (animal) soul and the intellectual soul. Some people used Aristotle to say that humans therefore have three souls. Others said that the intellectual soul is created by human semen.

Aquinas denied both positions. He said that a material cause (semen) cannot cause a spiritual product. The intellectual soul (personhood) is directly created by God “at the end of human generation.” This intellectual soul supplants what had preceded it (nutritive and sensory animation). So Aquinas denied that personhood arose at fertilization by the semen. God directly infuses the soul at the completion of human formation.

Much of the debate over abortion is based on a misconception -- that it is a religious issue, that the pro-life advocates are acting out of religious conviction. It is not a theological matter at all. There is no theological basis for defending or condemning abortion. Even popes have said that the question of abortion is a matter of natural law, to be decided by natural reason. Well, the pope is not the arbiter of natural law. Natural reason is.

John Henry Newman, a 19th century Anglican priest who converted to Catholicism, once wrote that “the pope, who comes of revelation, has no jurisdiction over nature.” The matter must be decided by individual conscience, not by religious fiat. As Newman said: “I shall drink to the pope, if you please -- still, to conscience first, and to the pope afterward.”

If we are to decide the matter of abortion by natural law, that means we must turn to reason and science, the realm of Enlightened religion. But that is just what evangelicals want to avoid. Who are the relevant experts here? They are philosophers, neurobiologists, embryologists. Evangelicals want to exclude them because most give answers they do not want to hear. The experts have only secular expertise, not religious conviction. They, admittedly, do not give one answer -- they differ among themselves, they are tentative, they qualify. They do not have the certitude that the religious right accepts as the sign of truth.

So evangelicals take shortcuts. They pin everything on being pro-life. But one cannot be indiscriminately pro-life.

If one claimed, in the manner of Albert Schweitzer, that all life deserved moral respect, then plants have rights, and it might turn out that we would have little if anything to eat. And if one were consistently pro-life, one would have to show moral respect for paramecia, insects, tissue excised during a medical operation, cancer cells, asparagus and so on. Harvesting carrots, on a consistent pro-life hypothesis, would constitute something of a massacre.

Opponents of abortion will say that they are defending only human life. It is certainly true that the fetus is human life. But so is the semen before it fertilizes; so is the ovum before it is fertilized. They are both human products, and both are living things. But not even evangelicals say that the destruction of one or the other would be murder.

Defenders of the fetus say that life begins only after the semen fertilizes the egg, producing an embryo. But, in fact, two-thirds of the embryos produced this way fail to live on because they do not embed in the womb wall. Nature is like fertilization clinics -- it produces more embryos than are actually used. Are all the millions of embryos that fail to be embedded human persons?

The universal mandate to preserve “human life” makes no sense. My hair is human life -- it is not canine hair, and it is living. It grows. When it grows too long, I have it cut. Is that aborting human life? The same with my growing human fingernails. An evangelical might respond that my hair does not have the potential to become a person. True. But semen has the potential to become a person, and we do not preserve every bit of semen that is ejaculated but never fertilizes an egg.

The question is not whether the fetus is human life but whether it is a human person, and when it becomes one. Is it when it is capable of thought, of speech, of recognizing itself as a person, or of assuming the responsibilities of a person? Is it when it has a functioning brain? Aquinas said that the fetus did not become a person until God infused the intellectual soul. A functioning brain is not present in the fetus until the end of the sixth month at the earliest.

Not surprisingly, that is the earliest point of viability, the time when a fetus can successfully survive outside the womb.

Whether through serendipity or through some sort of causal connection, it now seems that the onset of a functioning central nervous system with a functioning cerebral cortex and the onset of viability occur around the same time -- the end of the second trimester, a time by which 99% of all abortions have already occurred.

Opponents of abortion like to show sonograms of the fetus reacting to stimuli. But all living cells have electric and automatic reactions. These are like the reactions of Terri Schiavo when she was in a permanent vegetative state. Aquinas, following Aristotle, called the early stage of fetal development vegetative life. The fetus has a face long before it has a brain. It has animation before it has a command center to be aware of its movements or to experience any reaction as pain.

These are difficult matters, on which qualified people differ. It is not enough to say that whatever the woman wants should go. She has a responsibility to consider whether and when she may have a child inside her, not just a fetus. Certainly by the late stages of her pregnancy, a child is ready to respond with miraculous celerity to all the personal interchanges with the mother that show a brain in great working order.

Given these uncertainties, who is to make the individual decision to have an abortion? Religious leaders? They have no special authority in the matter, which is not subject to theological norms or guidance. The state? Its authority is given by the people it represents, and the people are divided on this. Doctors? They too differ. The woman is the one closest to the decision. Under Roe vs. Wade, no woman is forced to have an abortion. But those who have decided to have one are able to.

Some objected to Karl Rove’s use of abortion to cement his ecumenical coalition, on the grounds that this was injecting religion into politics. The supreme irony is that, properly understood, abortion is not even a religious issue. But that did not matter to Rove. All he cared about was that it worked. For a while.

Garry Wills is the author of numerous books, most recently “Head and Heart: American Christianities,” from which this article is adapted.