Editor's note: ERLC and Focus on the Family are hosting the first ever Evangelicals for Life event next year in Washington DC on January 21-22nd, featuring Russell Moore, Roland Warren, David Platt, Eric Metaxes, Kelly Rosati, Ron Sider and others.

By now, we’re familiar with the chilling scenes from the secretly-recorded videos about Planned Parenthood. Of the countless questions that remain unanswered about these videos and the issues they raise, there is at least one that seems to cry out for an answer: By what kind of moral reasoning are these  actions being justified?

This is not a rhetorical question intended to incite outrage (although outrage is certainly an appropriate response to the killing of unborn babies). It is a meaningful question intended to lay bare the ethical disparity between those who defend the rights of unborn children and those who think it’s legitimate to kill them.

Let's be clear: the question is not a matter of whether the unborn child is a human being. Instead, the question is what gives that human the right to life? How a person answers that question wields consequences that reach beyond the gestational period — into infancy and even old age.

Of course, I don’t claim to know what moral reasoning is happening (or not happening) inside the heads of abortionists. But I am aware that the moral justification for these kind of killings has been successfully popularized by a professor of bioethics at Princeton University. The author of fifteen books (and coauthor and coeditor of many more), he has been considered the most influential living ethicist today. Not long ago, I began to study his writings. I discovered his reasoning to be clear and consistent, his writing style compelling and his conclusions — horrifying.

His name is Peter Singer, and Planned Parenthood — conducting an average of 300,000 abortions every year — has been vigorously advancing his ideals.

The moral reasoning that justifies the killing of babies takes three simple steps, which Singer explains in his book Practical Ethics:

[Step One] The fact that a being is a human being, in the sense of a member of the species Homo sapiens, is not relevant to the wrongness of killing it; [Step Two] it is, rather, characteristics like rationality, autonomy, and self-consciousness that make a difference. Infants lack these characteristics. [Step Three] Killing them, therefore, cannot be equated with killing normal human beings, or any other self-conscious beings.

Step One: Discard the sanctity of human life

The first step requires discarding Scripture’s conception of what it means to be a “person” (by “person” we mean someone whose life is uniquely valuable). According to Scripture, someone possesses personhood simply because he or she is human — the only species created in the image of God (Genesis 9:6). From this simple fact, we derive the doctrine of the sanctity of life — that human life ought to be defended in a way that uniquely differs from the lives of other creatures. But in order to justify the killing of an unborn human, this doctrine must be rejected. Indeed, throughout his writings, Singer radiates his disgust for the idea of the sanctity of human life. To him, it is “untenable” and should be “abandoned.”

Step Two: Define “personhood” in terms of rationality, autonomy and self-consciousness

Having done away with the Scripture’s conception of personhood, the next step is a bit trickier — to come up with a different criteria for what it means to be a person. Singer’s solution is that “characteristics like rationality, autonomy and self-consciousness” should serve as the criteria for personhood. Anyone who is unfortunate enough to lack these characteristics doesn’t have the status as a person, and therefore, doesn’t have the unique right to live.

This bold redefinition forces a dramatic shift for ethical choices since it allows some animals (the great apes, chimpanzees, and perhaps some whales and dolphins) to join the ranks as persons. On the other hand, it excludes some humans, such as the profoundly retarded, those in a persistent vegetative state, and even healthy infants.

Step Three: Assert that killing unborn babies is morally justifiable.

Step three is barely a step at all. It’s the logical conclusion of steps one and two. Singer puts it concisely: “Killing [infants], therefore, cannot be equated with killing normal human beings, or any other self-conscious beings.” Or, as he states elsewhere: “The main point is clear: killing a disabled infant is not morally equivalent to killing a person. Very often it is not wrong at all.”

Up to what age may an infant be justifiably killed? Since his definition for personhood is rather arbitrary, so is the age up to which an infant may be justifiably killed. Singer suggests up to 28 days after birth.

Besides this horrendous conclusion, we should be aware that this moral reasoning leads inevitably to other forms of killing, including mass murder. Of course, Singer anticipated this “slippery slope” objection and spends several pages arguing why genocide need not follow from his logic. Near the end of the chapter “Taking Life: Humans,” he writes, “All of this is not to deny that departing from the traditional sanctity-of-life ethic carries with it a very small but nevertheless finite risk of unwanted consequences.”

In the time since that sentence was published, the “very small but nevertheless finite risk of unwanted consequences” has been the lives of  hundreds of thousands of human babies. If that’s not mass murder, I don’t know what is.

How does the killing of babies get justified? It starts when we jettison Scripture’s doctrine of the sanctity of life.

After that, it’s a free-for-all. It just depends on who has the power to say who gets to live and who doesn’t.