When the story broke in January that scientists at the biotechnology firm Stemagen had created a human embryo by cloning an adult skin cell, the chief executive, Samuel Wood, was interviewed about the fact that it was his skin cell that had been used to clone the embryo. Wood was asked what it was like to look at embryos that were replicas of himself. He responded, “I have to admit, it’s a very strange feeling. It is very difficult to look at an embryo and realize it is what you were a few decades ago.”

Robert George and Christopher Tollefsen would appreciate Wood’s honesty-if not his cavalier attitude toward embryos-because their new book is essentially a philosophical defense of the claim that human life begins at conception. The adjective “philosophical” is important here, for the authors note that such a view of the embryo is frequently dismissed as inherently religious and thus unsuitable as a basis for public reasoning. This book demonstrates that a vigorous nonreligious defense of the dignity of the embryo is possible; indeed, future debate about embryo status, whether philosophical or theological, will need to engage this volume.

According to George and Tollefsen, to deny the claim that the embryo deserves full moral respect at every stage of its life, one must reject one or more of the following claims: The early embryo is a human being; human beings are persons; all human beings deserve full moral respect. The bulk of this book is devoted to showing why rejecting any of these claims is a mistake.

After a chapter on why the embryo-experimentation debate is important, the book moves to examine the scientific facts of embryological development. This may seem like an odd way to begin, but the facts of embryological development are crucial to the main philosophical argument of the book. In particular, George and Tollefsen insist that one of the central questions in the debate over the moral status of the embryo is: “When is there a single biological system with a developmental trajectory, or active developmental program, toward the mature stage of a human being?” Their answer is clear and concise: The definitive point is “shortly after the union of sperm with oocyte.”

The fact that from fertilization onward-or, in the case of a cloned embryo, from nuclear transfer onward-we have a unified and self-integrating organism capable of self-directed growth means that fertilization marks the beginning of a new human being. Thus, to deny that a new human being comes into existence at conception is to deny the basic scientific facts of reproductive biology and embryological development.

What of the two other claims-namely, that human beings are persons or that all human beings deserve respect? Is there reason to believe that these claims cannot be rationally rejected? George and Tollefsen argue that the rejection of these claims cannot be justified philosophically and the chapters in which they make this case engage most of the major views and figures in the debate.

They argue that to reject either of these claims involves embracing one or another form of dualism, or requires the use of morally arbitrary criteria to justify the exclusion of embryos from our moral community. These criteria, they claim, are morally irrelevant, like skin color or gender. In developing this case, the authors rely on the language of “substance,” “essence,” and “nature,” and insist that we must distinguish between what sorts of beings we are essentially and what sorts of beings we are accidentally. Because we are essentially beings who have a natural capacity for reason and freedom, and because at every stage of our development we are such beings, embryos are not just potential life but actual human life, with a radical capacity for freedom and reason, which must be respected and protected.

This is an important book that deserves (and repays) a careful reading. It is also a deeply flawed volume. The book opens, for example, with an “inspirational” tale of how Noah Benton Markham had been trapped in a New Orleans hospital during Hurricane Katrina and rescued by the joint effort of the Illinois conservation police and Louisiana state police. As we read on, we learn that Noah was “one of the youngest residents of New Orleans to be saved from Katrina,” and that, had it not been for the “heroism” of the police officers, fourteen hundred other persons in Noah’s situation might have perished as well. What should we make, then, of the fact that “Noah” turns out to have been an embryo in a canister of liquid nitrogen and that what the police “rescued” were the vats of cryopreserved embryos?

I confess that my own reaction here is that the personal, relational language of “rescue” and “heroism,” of Noah being “trapped,” etc., is deeply out of place. But the more important concern is that this opening vignette sets the stage for an introductory chapter that is highly polemical. We read, for example, that Proposition 71 in California, which established funds for state-sponsored embryonic stem-cell research, created an industry that “is centrally about: the production and destruction of human beings in the earliest stage of development.” By page 10, embryonic stem-cell research is being compared with the Nazi and Tuskegee experiments and therapeutic cloning is compared to killing mentally retarded infants for their organs.

Fortunately, much of the rest of the work is free from this kind of ideological mud slinging, but not all of it is. And the problem here is not just that some readers may be troubled by the rhetoric. The difficulty is that it is nearly impossible to match concrete recommendations about the disposition of embryos with this kind of rhetoric. Consider, for example, the three specific recommendations George and Tollefsen offer at the conclusion of this volume.

First, the United States should prohibit embryo-destructive research. Second, the United States should significantly increase funding for stem-cell research that does not involve the destruction of embryos. Third, the United States should regulate in-vitro fertilization (IVF) clinics so that couples will not be permitted to create more embryos than they can reasonably expect to bring to term.

Perhaps the first two recommendations are consistent with a position that equates embryonic stem-cell research with the experiments of the Nazi doctors, but the third is not. If the earliest human embryo is a person who deserves full moral respect and protection, why should we allow IVF at all? Arguably, IVF owes its very existence to the kind of embryo experimentation that George and Tollefsen decry. Even if one does not believe that IVF is still largely experimental, why should we accept the rather bland recommendation that couples not be permitted to create more embryos than they can bring to term, when creating any IVF embryos involves complicity with what, on George’s and Tollefsen’s view, has to be considered a kind of atrocity?

In the end, George’s and Tollefsen’s arguments prove too much. If applied consistently, their reasoning should lead them to condemn IVF altogether and to advocate for a massive technological and medical effort to save all the embryos that expire from natural causes. That they do not rule out IVF or suggest that we allocate resources to find ways to lower the rate of natural embryo loss suggests just how difficult it is to have a consistent position on embryo status. George and Tollefsen are not unique in being inconsistent in this regard, but this inconsistency would be easier to excuse if they had avoided the Manichaean rhetoric to which they occasionally succumb when pointing out the problems in their opponents’ positions.

What we need in the debate over embryos is less emotionally charged rhetoric and greater care in developing and defending our respective views on the moral status of the embryo. That care would include acknowledging the weaknesses of our own position as well as those of others. This volume forcefully and carefully articulates and defends a view of the embryo that has many supporters; it is thus a valuable work. It would be even better if the rhetoric were toned down and George and Tollefsen acknowledged the problems for their position even as they press its undeniable strengths.