It is an interesting, difficult, and manifestly important question: “When does a new human being begin to exist?” Many crucial moral, political, and legal matters turn upon the answer. But this question is distinct from a further one: “How should we go about answering this important, difficult, and interesting question?” This question about inquiry is raised by Mathew Lu in a recent Public Discourse essay.

Lu’s essay is provocative, for it suggests that pro-lifers are not entirely correct in believing that scientific inquiry can answer the question of when the life of a human being begins. Lu mentions Senator Marco Rubio as a case in point. Rubio told Sean Hannity that the “science is settled, it’s not even a consensus, it is a unanimity, that human life begins at conception.”

I suppose Rubio could also have pointed to Robert George and me. Early in our book, Embryo: A Defense of Human Life, we write:

Embryo science tells us two important things about human embryos: what they are and when they begin. It tells us that human embryos are human beings at a certain (very early) developmental stage and that in the vast majority of cases, those human beings begin at conception, the initiation of a new single-celled organism after the fertilization of an egg by a sperm.

Start your day with Public Discourse

Sign up and get our daily essays sent straight to your inbox.

George and I were careful to frame our discussion of scientific knowledge in a very limited set of terms. Prominent among them was the term human being. Human beings are entities we are all familiar with: we meet them and speak with them regularly, and many, though perhaps not enough, will eventually read our book. Also important was the term human embryo. For much of human history, the object denoted by that term was not available for much empirical investigation. As a result, inquiry into prenatal development was often hopelessly speculative. But embryos, human and otherwise, are now available for empirical scientific scrutiny.

Human Beings and Human Embryos

Given that we have two entities, one easily observable, the other observable given the appropriate equipment, it was natural to ask what the relationship was between the two: are human embryos human beings, members of the same biological species as these beings we converse with every day? And when, as I will discuss below, the answer was given that yes indeed, human embryos are human beings, it was natural as well to ask when those embryos began to exist, for answering that question would suffice to answer the question “When does a new human being begin to exist?”

Notice that we did not ask when life begins, or even when human life begins. That question is vague and ambiguous. Human life began some two hundred thousand years ago, a fact fairly irrelevant to the concerns of our book. Nor did we ask what the criteria are for ascertaining whether a living thing exists. That question might be understood in various ways. Our questions were quite simple, and can be asked by anyone familiar with human beings and human embryos.

But not everyone is equally qualified to answer these questions. Questions about the human embryo and its beginnings seem to be questions of biology, and biologists seem to be competent authorities in answering them. And biologists have indeed answered those two questions in a way that approaches universal consensus: human embryos are human beings, and human embryos, when not the product of monozygotic twinning or human cloning, come into existence at conception, when a sperm penetrates an ovum, both cease to exist as haploid gametes, and a new, whole, human individual comes into existence.

But Lu argues in his essay that the science is not enough to return these verdicts; rather, a metaphysics of life is first necessary, one that gives us a philosophical account of what life is. As Lu writes, “Once you have a metaphysical account of life, then empirical embryology can tell you whether the relevant conditions are met.”

To paraphrase Lu, I believe that “this claim is not exactly false.” It is not exactly false because the work done by embryologists in answering these questions about the human embryo implicitly relied on answers to other questions about embryonic life, which were not always articulated explicitly by those biologists.

Moreover, both these implicit questions and their answers bear very clearly on our understanding, both pre-philosophical and philosophical, of concepts such as life, living being, organism, individual, and the like. Thus, philosophical reflection on biological topics lies very close to the work of actual biologists, and works with concepts that, explicitly or implicitly, guided the work of those biologists. Yet it is, I am arguing, the work of biologists first and foremost to answer the questions about embryos that I have identified, and they simply do not need the guidance of philosophy in order to get there.

Biologists’ Implicit Questions

What were the questions that guided biologists’ approach to the human embryo? Their inquiry required attention to at least the following:

First, does the embryo act like a single organism?

The biological sciences are not concerned only with “life”—they are concerned with living things. The identification of some things as living requires the ability and willingness to see certain clusters of matter and energy as agents of processes that are end-directed. As Lu puts it, living things have “immanent causal powers” that are “directed to their good.” This is true, and it admits of much philosophical analysis. But this analysis was not needed by those biologists who discovered that the embryo does indeed act as an individual organism manifesting the life form of a particular species, and not as a collection of individuals on their way to becoming some one thing.

Second, biologists asked whether the embryo possessed a distinction of function, a division of labor, such that it could be understood to have parts. For if the embryo was simply, as some allege, an undifferentiated mass of cells, it could not be a whole living organism.

Thus, biologists sought out the sort of knowledge eventually obtained by Magdalena Zernicka-Goetz, of Cambridge University, who dyed the first two cells of the embryo, one red, one blue, and then tracked their descendants into the blastocyst. As an article in Nature puts it,

One cell usually gave rise to the region containing the ICM, the other to the region largely destined to make the placenta and other supporting tissues . . . Zernicka-Goetz’s conclusion is that the first division of the egg influences the fate of each cell and ultimately, all the tissues of the body. “There is a memory of the first cleavage in our life,” says Zernicka-Goetz.

The essay in Nature concludes: “What is clear is that developmental biologists will no longer dismiss early mammalian embryos as featureless bundles of cells.”

Biologists were concerned with this unity in diversity precisely because they recognized it as characteristic of living natural beings, in whom the unfolding of such unity in diversity is, from the beginning, the way that individuals in a complex life form develop; they do not spontaneously come to be from mere aggregated biological stuff. There is much for philosophers to mine here, but the insight is constitutive of biological knowledge, without which there would be no such discipline.

Third, biologists investigated whether the coordinated functioning of the different parts of the embryo was recognizably ordered toward the good of the whole embryo.

One way in which this question was addressed was through study of the embryos’ regulative nature as seen, for example, in the ability of cells to shift their developmental trajectory when moved from one part of the embryo to another. Such adaptations are crucial if the embryo is to preserve a normal structure. They were initially discovered by early twentieth-century experimental embryologists, such as Wilhelm Roux, Hans Dreisch, and Hans Spemann.

Science, Philosophy, and Definite Answers

Now, to repeat, all these questions admit of further philosophical exploration, and they raise any number of interesting and important questions. And it is, in some sense, the job of philosophy to articulate the questions that motivated certain aspects of embryological inquiry. But it was the biologists who carried out that inquiry; they were guided sometimes explicitly, sometimes implicitly by these and other questions. Philosophy’s work is largely subsequent to this endeavor.

The same was true when embryologists pushed the inquiry further to the question of when the embryo began to exist. That inquiry was likewise guided by the desire to find when there was a single whole individual possessed of the “immanent causal powers” necessary to be a self-integrating and developing being, as the zygote, from the moment of sperm penetration, is. But this understanding of what it meant to be a living organism was internal to biology. Indeed, there could hardly be a discipline of biology without this understanding.

Philosophy’s work is never over, and philosophers will continue to analyze, interpret, and theorize about embryology in different and incompatible ways. That is part of the nature of philosophy. Although it is oriented to truth, it does not result in the same kind of convergence and consensus that the sciences do. If philosophy were truly a necessary prerequisite for embryological study, we would never have definite answers to our questions.

Why science does, and philosophy does not, admit of convergence is itself a difficult philosophical question. But that difference itself suggests that in the order of knowing, science comes first and does not need to wait on philosophy’s pronouncements to investigate what are clearly questions appropriate to its domain: what is the nature of this biological entity (the human embryo)? And when did it come into being?

Rubio was on solid ground. All philosophers (and, for that matter, government officials) would be wise to recognize that and to take the scientific consensus he invoked as our starting point. By doing so, philosophers can be assured that our speculative and analytical work will not be untethered from the reality of human life.