George Tiller, a doctor and abortion provider in Kansas, was shot and killed outside his church on Sunday. The large majority of people on either side of the abortion debate are understandably horrified by an event like this. But it sets up a rhetorical dilemma for anyone who takes seriously the claim that abortion is murder. If George Tiller really was a "baby killer" comparable to Hitler and Stalin, it's difficult to express unmitigated sadness at his murder. So we get Randall Terry, founder of Operation Rescue, admitting regret -- but only that Tiller was a mass murderer who "did not have time to properly prepare his soul to face God." On those rare occasions when they attempt to actually talk to each other, people on opposite sides of the abortion debate usually end up talking past each other. Supporters of abortion rights speak in the language of the autonomy of the mother, and her right to control her own body: "If you don't like abortion, don't have one." Opponents of abortion speak in terms of the personhood of the fetus. (Yes, Dr. Seuss's Horton Hears a Who! -- "A person's a person, no matter how small" -- is used to teach this point to Catholic children, over Theodor Geisel's objections.) Opposition to abortion rights can also be a manifestation of the desire to control women's sexuality, but let's concentrate on those whose opposition is grounded in a sincere moral belief that abortion is murder. If someone believes that abortion really is murder, talk of the reproductive freedom of the mother isn't going to carry much weight -- nobody has the right to murder another person. Supporters of abortion rights don't say "No, this is one case where murder is completely justified." Rather, they say "No, the fetus is not a person, so abortion is not murder." The crucial question (I know, this is not exactly an astonishing new insight) is whether a fetus is really a person. I have nothing original to add to the debate over when "personhood" begins. But there is something to say about how we decide questions like that. And it takes us directly back to the previous discussion about marriage and fundamental physics. The upshot of which is: how you think about the universe, how you conceptualize the natural world around us, obviously is going to have an enormous impact on how you decide questions like "When does personhood begin?" In a pre-scientific world, life was -- quite understandably -- thought of as something intrinsically different from non-life. This view could be taken to different extremes; Plato gave voice to one popular tradition, by claiming that the human soul was a distinct, incorporeal entity that actually occupied a human body. These days we know a lot more than they did back then. Science has taught us that living beings and non-living objects are the same kind of things, deep down; we're all made of the same chemical elements, and all of our constituents obey the same laws of Nature. Life is complicated, and rich, and fascinating, and not very well understood -- but it doesn't obey separate rules apart from those of the non-living world. Living organisms are just very complicated chemical reactions, not vessels that rely on supernatural essences or mystical élan vital to keep them chugging along. Except "just" is a terribly misleading adverb in this context -- living organisms are truly amazing very complicated chemical reactions. Knowing that we are made of the same stuff and obey the same rules as the rest of the universe doesn't diminish the value or meaning of human life in any way. There is a temptation in some quarters to forget, or at least ignore, the improved understanding of the world that science has given us when it comes to address moral and ethical questions. Part of that is a healthy impulse -- science doesn't actually tell us how to distinguish right from wrong, nor could it possibly. Science deals with how the world works, not how it should work, and despite centuries of trying it remains impossible to derive "ought" from "is." But at the same time, it would be crazy not to take our scientific understanding of the world into consideration when we reflect upon moral questions. If you think of a fetus as part of an ongoing complicated chemical reaction, it should come as no surprise that you might reach very different conclusions from someone who thinks that God breathes the spirit of life into a fertilized ovum at the moment of conception. That's why it's equally crazy to believe that science and religion are two distinct, non-overlapping magisteria that simply never address the same questions. That bizarre perspective was advanced by Stephen Jay Gould in Rocks of Ages, but if you read the book carefully you find that his definition of "religion" is simply "moral philosophy." Which is not what the word means, or how people use it, or how actual religious people think of their beliefs. Religion makes claims about the real world, and some of those claims -- not all -- can be very straightforwardly judged by the criteria of science. We do not need to invoke spirits being breathed into fertilized eggs in order to understand life, for example. And the fact that science has taught us so much about the workings of the world has enormous consequences for how we should think about moral and ethical questions, even if it can't answer such questions all by itself. For example, science is powerless to tell us when "personhood" begins -- but it tell us something very crucial about how to go about answering that question. In particular, it tells us that there is no magical moment at which an incorporeal soul takes up residence in a body. Indeed, the concept of a "person" is not to be found anywhere in the natural world; it's a category that is convenient to appeal to as we try to make sense of the world. But there is not, as far as science is concerned, any right or wrong answer to the question of when the life of a person begins -- from Nature's point of view, it's just one chemical reaction after another. At this point, a lot of impatient people declare that morality and ethics are simply impossible in such a world, and storm out in frustration. But this is the world in which we actually live, so storming out is not a productive response. Morality and ethics are possible, but they're not to be found in Natural Law -- they are the creation of human beings, reasoning together on the basis of their shared feelings and experiences. Human beings are not blank slates, nor are they immutable tablets; we are born into the world with certain wants and desires and natural reactions to events, and those feelings can adapt and change over time in response to learning and reasoning. So we get together, communicate, understand that not everyone necessarily agrees on how the social world should be organized, and try to negotiate some sort of mutual compromise. (Or, alternatively, try to impose our will by force. But I like the mutual compromise approach better.) That's how the world actually works. "The moment when a fetus begins to accrue the rights we bestow on post-birth persons" is something that we, as a society, have to decide; the answer is not to be found in revelation, or in faith, or in philosophical contemplation of the nature of the soul, or for that matter in the natural world. This starting point is not necessarily prejudicial to what the final answer may be; I can certainly imagine a group of people coming together and agreeing that newly-conceived fetuses should be granted all the rights of any person. I would argue against them, on the basis that the interests of an autonomous and fully conscious mother should weigh much more heavily than those of the proto-person they carry. But I can't say that they are unambiguously wrong in the same way that an erroneous claim about logic or even the empirical world can be said to be "wrong." If the social and political arrangement of a group puts stress on the autonomy of its individual responsible members (which ours does, and I like it that way), deciding what the criteria are for being judged an "individual responsible member" is of primary importance. Who gets to vote? Who gets to drive a car? Who decides when to unplug the respirator? Who is of "sound mind"? Who is a person? These are all hard questions with no cut-and-dried answers. But we can be fooled into thinking that some of the answers are pretty straightforward, if we believe in outdated notions of spirits being breathed into us by God. There are many reasons why it's incoherent to think of science and religion as simply separate and non-overlapping. They are different, but certainly overlapping. The greatest intellectual accomplishment of the last millennium is the naturalistic worldview: everything is constructed of the same basic building blocks, obeying the same rules, without any recourse to the supernatural. Appreciating that view doesn't tell us how we should behave, but failing to appreciate it can very easily lead people to behave badly.