Toward the beginning of Challenging Nature, Lee Silver sounds a defensive note: “I do not claim that all expressions of spirituality are harmful or bad. Nor do I think that all biotech applications are inherently good, ethical, or risk-free.” It was wise of the Princeton molecular biologist to include those two sentences. The reader might otherwise have been misled by every other sentence in the book into thinking that Silver considers religion a regrettable—though regrettably ineradicable—feature of human society, and that no serious limits should be placed on biotechnology.

Silver contends that our debates over biotechnology are distorted by irrational religious beliefs—and, worse, by the failure of participants in the debate to come clean about those beliefs: “Catholic and evangelical Christians strive to appear rational and scientific in their opposition to embryo research.” At the same time, behind environmentalists’ advocacy of “natural” food and medicine lies a nature-worship that they have concealed even from themselves. Silver’s book does not address these people directly. “Although fundamentalist faith-based extremists from the right and left will probably remain beyond engagement,” he writes, “their stealth extremism can be revealed to everyone else.”

Silver’s argument that religious fundamentalists conceal their true beliefs proceeds along two tracks. On one of them, he notes that many of the people who make secular-sounding arguments are in fact religious. He seems to think that such people are being deceptive whenever they make their arguments without disclosing their religious commitments. Were this standard applied evenhandedly, Silver would be unable to write an op-ed on biotechnological issues without mentioning up front that he is an “extremely skeptical agnostic deist” (as he describes himself in an endnote), and his arguments could be dismissed as a product of his theology. The only way to avoid that conclusion would be to assume that religious people are less rational, or less honest, than the irreligious (and the agnostic deists). Silver may take this view; he refers casually at one point to “mystics, religionists, spiritualists, and other charlatans.” But the assumption is not obviously true, and Silver provides no reasons for believing it.

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The President’s Council on Bioethics, chaired by Leon Kass through 2005 and currently headed by Edmund Pellegrino, especially exercises Silver. He considers a majority of its members to hold “extreme” views borrowed from the Judeo-Christian tradition. It is hard to know what to make of this charge. Although Silver does not mention it, only a minority of the council favors a ban on cloning, which arguably puts it to the left of the population at large. And by any reasonable standard, being influenced by Judaism and Christianity does not make one an extremist. That Silver thinks it does may say more about him than about his targets.

He does real injustice to council member Diana Schaub. Twice he truncates a quote of hers to make it sound as though she were saying that research that destroys embryos is morally worse than slavery. The full quote, presented only in an endnote, makes it clear that all she is saying is that defenders of the embryo have available to them an argument that defenders of the slave did not: namely, that all of us once were embryos. (Obviously, not all of us once were slaves.)

On the second track, Silver claims that religious believers’ secular-sounding arguments are so clearly defective that they must be camouflage for undisclosed theological concerns. If the arguments really were defective, then the demonstration of hidden motives would be superfluous. But he does not refute the arguments, let alone prove that they are made in bad faith.

Silver himself makes several reductio ad absurdum arguments: for example, his idea is that those who think a human embryo is a human being with moral worth should think the same of all living human cells, since they too are alive and genetically human. Oddly, he sees no difference between a whole organism and a part of one. But pro-lifers need not join him in regarding embryonic stem cells as equivalent to the human embryos used to produce them. Scientists may be able to create embryos by manipulating certain cells or combining them with other biological entities; but this possibility does not mean those cells should be treated as though they are already embryos before the scientists touch them, as Silver also thinks. Tumors that are made up of living human cells are also not organisms—again, a distinction he misses.

The phenomenon of twinning poses a problem, in Silver’s mind, for pro-lifers: how can an embryo be held to be an individual human being if it can split into two embryos? To see his way out of this problem, he should consider his own advocacy of cloning human embryos for research and therapeutic uses. The goal is to treat a patient by taking one of his cells and using it to create an embryo that is genetically his identical twin. If this procedure were done on a patient, it would not call into question his own status as an individual human being with a right not to be killed. The fact that a twin human embryo can be spontaneously created from an embryo shouldn’t call its status into question, either.

The proposition that a human embryo is a human being, Silver complains, is unscientific because it is non-falsifiable. It is true that truisms aren’t falsifiable; but unless science can proceed without definitions, it will have to live with them. Silver trips over properties of logic more than once. He spends a lot of time on Princeton philosopher Robert P. George’s claim that “[a] thing either is or is not a whole human being.” He takes George to be denying the possibility that scientists could create a being that was, say, partly human and partly chimp. It is a mistaken conclusion, but one that hints at a radicalism that Silver is himself concealing.

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At a few points in the book, he mentions—always with seeming approval, but never with explicit endorsement—the views of Peter Singer (who blurbed the book). Singer notoriously holds that an organism’s mere possession of biological humanity confers no worth on it. Non-human animals with an awareness of their own existence over time may have worth, while humans without it—fetuses and newborns, for example, or adults in a “persistent vegetative state”—may not. On this view, human rights do not, strictly speaking, exist. (Imagining that they do constitutes “speciesism”: a neologism of Singer’s that is barbarous in more than one sense of the word.)

Silver goes Singer one better: he seems to deny that human beings exist. Evolutionary theory debunks the notion that there are clear boundaries between species. There are only populations with overlapping traits that we classify together for convenience. When we start creating human/non-human hybrids, Silver writes, we will have to draw “arbitrary” lines to determine which ones will have rights. He breezily approves the prospect.

In one of his book’s more baffling passages, Silver argues—well, let him speak for himself:

The Darwinian idea of continuous evolution leads logically to a parallel developmental perspective in which an embryo begins as a single-cell nonhuman being that morphs through a fuzzy partial human being stage, and then morphs into a newborn baby. If no nonarbitrary starting point exists for human beingness during evolution, then no nonarbitrary starting point need exist during embryo development either. The only basis for rejecting this argument is religious—the traditional Christian belief in an indivisible, absolute, unchanging human soul.

The only basis? Silver leaves out the possibility that an agnostic could dislike non sequiturs. (And what’s an “absolute” soul, by the way?)

One answer to Silver’s challenge is obvious: just as the existence of dusk does not eliminate the difference between night and day, the fact that hybrids—or evolutionary precursors of human beings—pose difficult problems of classification does not prove, or even suggest, that it is difficult to classify a human embryo as human. The Reverend Nicanor Pier Giorgio Austriaco has provided a deeper answer:

[T]he biological species concept “works.” It allows one biologist in Russia to do an experiment with his mouse of the species Mus musculus, while expecting to reproduce the results of another scientist in Singapore who had done her experiments with another mouse of the same species. But why does the species concept work? Why do the two scientists get the same results? Biological essentialism provides an answer where nominalism cannot—the biological species concept works because members of the same species actually do share a common essence that behaves in a reproducible manner when it is manipulated in the same way. Biological essentialism makes the biological task intelligible.

Many of Silver’s quarrels with evangelical and Catholic conservatives display a weak grasp of the material. Lifenews.com is not part of the “evangelical press.” The Catholic Church did not reach the conclusion that embryos should be protected from conception onward because of a belief that sperm were tiny human beings. The view that embryos have “a rational nature” is not “equivalent to saying that a single cell can think on its own and make sensible choices about its future life.” The word “goods” is not “a term used uniquely by adherents of natural law.” Pro-lifers have no ambition to deny other people’s rights “not to hold a Christian belief in the ensoulment of an early embryo”; they merely deny that people have a right to kill embryos on the basis of their beliefs. The proposed ban on cloning would not make it a crime to go overseas to get medical treatments based on cloning and then return to the U.S. The congressman who introduced that bill is named David, not Joseph, Weldon. And so, alas, on.

Silver is surely right that environmentalists confuse “natural” with good when it comes, for example, to the chemicals we encounter in everyday life. But does this confusion really prove that environmentalists are post-Christian nature-worshippers? Silver holds that natural law theorists are also guilty of identifying the good with the natural in a biological sense; but his analysis does not inspire confidence that he has dealt fairly with their arguments.

It was once popular to speak of “the two cultures.” In the one were the scientists, who knew little of the humanities; and in the other were the humanists, who knew little of the sciences. Lee Silver belongs to the first camp. He is an accomplished scientist, and his book is not without some interesting trivia. I had not known, for instance, that almost none of the molecules that compose a human body are still there a month later. Silver is at his best when he is teaching biology. The results are far less edifying when he commits philosophy, especially when he seems unaware that he is doing so.