For the complete text, including illustrations, introductory quote, footnotes, and commentary on the reaction to the originally published article see Billions and Billions.

If you deliberately kill a human being, it's called murder. If you deliberately kill a chimpanzee--biologically, our closest relative, sharing 99.6 percent of our active genes--whatever else it is, it's not murder. To date, murder uniquely applies to killing human beings. Therefore, the question of when personhood (or, if we like, ensoulment) arises is key to the abortion debate. When does the fetus become human? When do distinct and characteristic human qualities emerge?

We recognize that specifying a precise moment will overlook individual differences. Therefore, if we must draw a line, it ought to be drawn conservatively--that is, on the early side. There are people who object to having to set some numerical limit, and we share their disquiet; but if there is to be a law on this matter, and it is to effect some useful compromise between the two absolutist positions, it must specify, at least roughly, a time of transition to personhood.

Every one of us began from a dot. A fertilized egg is roughly the size of the period at the end of this sentence. The momentous meeting of sperm and egg generally occurs in one of the two fallopian tubes. One cell becomes two, two become four, and so on—an exponentiation of base-2 arithmetic. By the tenth day the fertilized egg has become a kind of hollow sphere wandering off to another realm: the womb. It destroys tissue in its path. It sucks blood from capillaries. It bathes itself in maternal blood, from which it extracts oxygen and nutrients. It establishes itself as a kind of parasite on the walls of the uterus.

So, if only a person can be murdered, when does the fetus attain personhood? When its face becomes distinctly human, near the end of the first trimester? When the fetus becomes responsive to stimuli--again, at the end of the first trimester? When it becomes active enough to be felt as quickening, typically in the middle of the second trimester? When the lungs have reached a stage of development sufficient that the fetus might, just conceivably, be able to breathe on its own in the outside air?

The trouble with these particular developmental milestones is not just that they're arbitrary. More troubling is the fact that none of them involves uniquely human characteristics--apart from the superficial matter of facial appearance. All animals respond to stimuli and move of their own volition. Large numbers are able to breathe. But that doesn't stop us from slaughtering them by the billions. Reflexes and motion are not what make us human.

Other animals have advantages over us--in speed, strength, endurance, climbing or burrowing skills, camouflage, sight or smell or hearing, mastery of the air or water. Our one great advantage, the secret of our success, is thought--characteristically human thought. We are able to think things through, imagine events yet to occur, figure things out. That's how we invented agriculture and civilization. Thought is our blessing and our curse, and it makes us who we are.

Thinking occurs, of course, in the brain--principally in the top layers of the convoluted "gray matter" called the cerebral cortex. The roughly 100 billion neurons in the brain constitute the material basis of thought. The neurons are connected to each other, and their linkups play a major role in what we experience as thinking. But large-scale linking up of neurons doesn't begin until the 24th to 27th week of pregnancy--the sixth month.

By placing harmless electrodes on a subject's head, scientists can measure the electrical activity produced by the network of neurons inside the skull. Different kinds of mental activity show different kinds of brain waves. But brain waves with regular patterns typical of adult human brains do not appear in the fetus until about the 30th week of pregnancy--near the beginning of the third trimester. Fetuses younger than this--however alive and active they may be--lack the necessary brain architecture. They cannot yet think.

Acquiescing in the killing of any living creature, especially one that might later become a baby, is troublesome and painful. But we've rejected the extremes of "always" and "never," and this puts us--like it or not--on the slippery slope. If we are forced to choose a developmental criterion, then this is where we draw the line: when the beginning of characteristically human thinking becomes barely possible.

It is, in fact, a very conservative definition: Regular brain waves are rarely found in fetuses. More research would help… If we wanted to make the criterion still more stringent, to allow for occasional precocious fetal brain development, we might draw the line at six months. This, it so happens, is where the Supreme Court drew it in 1973--although for completely different reasons.

Its decision in the case of Roe v. Wade changed American law on abortion. It permits abortion at the request of the woman without restriction in the first trimester and, with some restrictions intended to protect her health, in the second trimester. It allows states to forbid abortion in the third trimester, except when there's a serious threat to the life or health of the woman. In the 1989 Webster decision, the Supreme Court declined explicitly to overturn Roe v. Wade but in effect invited the 50 state legislatures to decide for themselves.

What was the reasoning in Roe v. Wade? There was no legal weight given to what happens to the children once they are born, or to the family. Instead, a woman's right to reproductive freedom is protected, the court ruled, by constitutional guarantees of privacy. But that right is not unqualified. The woman's guarantee of privacy and the fetus's right to life must be weighed--and when the court did the weighing' priority was given to privacy in the first trimester and to life in the third. The transition was decided not from any of the considerations we have been dealing with so far…--not when "ensoulment" occurs, not when the fetus takes on sufficient human characteristics to be protected by laws against murder. Instead, the criterion adopted was whether the fetus could live outside the mother. This is called "viability" and depends in part on the ability to breathe. The lungs are simply not developed, and the fetus cannot breathe--no matter how advanced an artificial lung it might be placed in—until about the 24th week, near the start of the sixth month. This is why Roe v. Wade permits the states to prohibit abortions in the last trimester. It's a very pragmatic criterion.

If the fetus at a certain stage of gestation would be viable outside the womb, the argument goes, then the right of the fetus to life overrides the right of the woman to privacy. But just what does "viable" mean? Even a full-term newborn is not viable without a great deal of care and love. There was a time before incubators, only a few decades ago, when babies in their seventh month were unlikely to be viable. Would aborting in the seventh month have been permissible then? After the invention of incubators, did aborting pregnancies in the seventh month suddenly become immoral? What happens if, in the future, a new technology develops so that an artificial womb can sustain a fetus even before the sixth month by delivering oxygen and nutrients through the blood--as the mother does through the placenta and into the fetal blood system? We grant that this technology is unlikely to be developed soon or become available to many. But if it were available, does it then become immoral to abort earlier than the sixth month, when previously it was moral? A morality that depends on, and changes with, technology is a fragile morality; for some, it is also an unacceptable morality.

And why, exactly, should breathing (or kidney function, or the ability to resist disease) justify legal protection? If a fetus can be shown to think and feel but not be able to breathe, would it be all right to kill it? Do we value breathing more than thinking and feeling? Viability arguments cannot, it seems to us, coherently determine when abortions are permissible. Some other criterion is needed. Again, we offer for consideration the earliest onset of human thinking as that criterion.

Since, on average, fetal thinking occurs even later than fetal lung development, we find Roe v. Wade to be a good and prudent decision addressing a complex and difficult issue. With prohibitions on abortion in the last trimester--except in cases of grave medical necessity--it strikes a fair balance between the conflicting claims of freedom and life.