When does life begin? Most people seem to have an opinion on the issue. After all, “When does life begin?” seems like a reasonable thing to ask. We see personhood as an all-or-nothing thing — something either is a person or it isn’t. If there are no degrees of personhood, there should be a clear moment when a new individual comes into existence, a thunderclap announcing a new life — “ensoulment,” if you will.

But it’s not the case. Every clear line of “where life begins” collapses under scientific inspection.

Let’s take a look at the candidates. Conception seems to be a popular choice for the beginning of life. As this is the moment when two genomes fuse to create a unique individual, maybe a new soul is created then.

But there are some problems with this idea. According to Steven Pinker in “The Blank Slate” (2002), the moment of conception is not a really a moment at all. Sometimes, more than one sperm penetrates an egg, and it takes time for the egg to push out the extra DNA. Also, even when only a single sperm enters an egg, their genes don’t mingle for at least a day. Then it takes another day for the new genome to control the embryo. So the “moment” of conception is really up to two days.

But even if it’s not really a moment, the point at which a new genome is created could still be the point of ensoulment. But now we have more problems. A new embryo can split and become twins, triplets, etc. Did the Dionne quintuplets scrape by with 1/5 of a soul apiece?

In addition, every cell of an eight-cell embryo is capable of becoming a full person. Does the embryo start with eight souls, and then lose some? Every singleton that is born deprives the world of an additional seven people. Perhaps we should require all women to deliver octuplets!

A weirder ensoulment dilemma happens with genetic chimeras, which are created when two embryos fuse to become one person. The resulting individual ends up with some cells of one genome and some of another. Does a genetic chimera have a double soul? One soul in some cells, and another soul in the rest?

Cloning throws one more wrench into the ensoulment debate. Dolly the cloned sheep was made with the nuclear DNA from an adult ewe’s mammary cell. (That’s actually why she was called “Dolly,” after Dolly Parton.) The mammary cell’s nucleus was transferred to the enucleated egg of a second sheep, which then developed into an embryo.

Did Dolly have the same soul as the donor of her DNA? Maybe sheep don’t have souls anyway, but consider this: Human cloning is already technically possible. Should it become a reality, every cell in a human body will have the potential to become a human being. When would a cloned human’s life begin?

Even with conventional non-clones, up to half of all fertilized eggs don’t implant into the uterus. They are flushed out of a woman’s body with her next period — she probably doesn’t even notice.

If we believe life begins at conception, this is the biggest public health crisis the world has ever faced! For every person born, one died in utero! Why isn’t there more research money going to save all these children? For me, this reflects that no one takes the idea that conception is the start of life too literally.

Are there other definitions for where life begins? The Catholic Church used to hold that the soul entered the body at “quickening” — that is, when a pregnant woman first felt the movement of her fetus. This isn’t a clear line either. A woman pregnant for the first time usually feels these movements at about 20 weeks’ gestation, while a woman who’s been pregnant before has more relaxed uterine muscles, and can feel movement as early as 14 weeks.

But a fetus is moving before the pregnant woman realizes what’s going on. Early fetal movement is often mistaken for hunger pangs or intestinal discomfort. Mistaking ensoulment for gas seems particularly funny.

So maybe quickening is out of the running. No one seems to give it much thought these days anyway. How about viability as the point where life begins? That is the basis for most of the policy decisions in the United States.

But the point of viability really only measures the state of medical technology. The world’s youngest surviving baby was recently born at 21 weeks, six days. Serious medical heroics kept her alive, but she won’t be the last baby in a parade of ever-younger preemies.

So where does all this leave us? With the uncomfortable realization that there is no thunderclap of ensoulment. We have to face the fact that personhood is a matter of degree. Life begins gradually, over several months. It’s decidedly unsatisfying.

A little more nuance in our understanding of personhood would bring some balance to the abortion debate. But even then, the science doesn’t deliver a clear policy.

Say we agree that a fetus is some fraction of a person. What then? Is it okay for a woman to have an abortion if the fetus is less than 50 percent of a person? Less than 40 percent? What if she’s carrying twins, and they are each 50 percent of a person? Is abortion then unacceptable, since together the fetuses sum to 100 percent of a person? Maybe twins could only be aborted at the developmental stage, where they were each 25 percent of a person? The situation gets ludicrous fairly quickly.

I hate to say it, but maybe in some cases, science can only take us so far.