I thank my colleagues Patrick Lee and Robert George for the time and effort that they put into reading and critiquing my recently published book, Challenging Nature.   Since I present a lengthy discussion of -- and challenges to -- their previously published views in the first half of my book, I was not surprised that they found much to criticize there.   I am disappointed that they had nothing to say about the second half of my book, where I direct my fire at "New Age secularists [who] rail against genetically modified crops."  I suspect that many NRO readers will find much to agree with in this portion of my writing (which elicited an enraged review in the American Scientist from the post-modern historian Nathaniel Comfort).

Before I respond to the main points of disagreement, I want readers to know that I wrote about leaders of the Catholic  Church in the U.S. with the utmost respect.   In describing my meeting with bishops who sit on the Committee on Science and Human Values of the U.S. Conference of Catholic Bishops, I wrote, "At the outset, I was impressed by their genuine inquisitiveness, their desire to understand the most recent advances in science, and their willingness to admit that important questions existed for which they had no answer."  As I have written, the thrust of my challenge to Lee and George was on scientific grounds, not theological ones.

Professors Lee and George think that human embryos are undisputedly human beings.  Millions of other people brought up in the Western religious tradition hold the same view, but what sets Lee and George apart is their claim that the "scientific facts" alone prove that human embryos are human beings.  Lee and George have read and heard challenges to their claim by scientists, writers, theologians, and other academics. Most notable are the able arguments presented by Ron Bailey in a previous NRO debate.   But Lee and George haven't budged, and I doubt there is anything I can say or write that will ever make them budge.    My perception of their attitude should make scientists suspicious because it suggests that Lee and George cannot conceive of an empirical experiment (with results that can be objectively verified) that would falsify their claim.  If this is true, their claim is not a scientific one.  If they've got an experiment, I'd like to hear the operational details.

Lee and George base their embryo-is-a-human-being claim mainly on two propositions.  One is that a human embryo has an "internal active disposition for self-directed development toward the mature stage of a human."   Since we know that a single cell can separate from a 4-cell embryo and develop into a separate human baby (on rare natural occasions), Lee and George would argue that this cell has the "internal active disposition"  that makes it a human being.  On the other hand, they argue that isolated embryonic stem (ES) cells are not human beings because they do not have this "internal active disposition."   But "internal active disposition" is not a term that has any meaning in the context of cellular or molecular biology.  If George and Lee want to claim otherwise, I'd like to know their perception of the molecular attributes that distinguish human being-cells from non-human-being, yet still fully viable and human, cells.

Those of us who have worked only with early-stage mouse embryos (which are essentially identical in size and appearance to human embryos) don't have the same qualms as colleagues in the human stem cell field do when it comes to speaking honestly about the relationship of embryos to stem cells.  In the summary of a meeting of mouse geneticists published in the journal Science, the reporter wrote, "the participants agreed that it would be most economical to avoid trafficking in live mice and instead decided to maintain the knockouts as embryonic stem (ES) cells: clumps of tissue that can be frozen down and later grown up into full-fledged mice." (Science 312: 1862-1866; June 30, 2006)    Furthermore, as I wrote in my book, human ES cells have already been differentiated into placenta.  This means that, in theory, the requirement for a second source of cells to reconstitute an embryo may be nullified, and ES cells -- all by their lonesome -- could develop into a fetus and human baby.  This discussion was conveniently left out of Lee and George's review.  But, if theory becomes practice, and ES cells can be grown directly into a fetus, at what point during continuous development from a bunch of cells to a fetus does a human being instantly appear? Tell me what the molecular correlates are for such an event.

The second proposition that Lee and George use in their appeal to science has been simply stated on many occasions by Professor George: "a thing either is or is not a human being."   Lee and George provide several arguments in support of this proposition.  One is that it is simply common sensical to most people.  I have no doubt that this observation is correct, but as many examples from science demonstrate, common sense is no predictor of truth.   Furthermore, it is not surprising that so many philosophers have also held this position.  Two millennia ago, in the absence of modern scientific knowledge and biomedical technology, Aristotle would certainly have convinced me of its veracity.  Modern psychologists have evidence that normal human brains are hard-wired to categorize everything into distinct classes (including human beings and non-human beings), and integer numbers -- so how do you count the number of human beings in each body shown to the right?

Scientific knowledge challenges this instinct on many occasions.   The most serious challenge, of course, came from Darwin whose theory of natural selection suggests that in the evolution of pre-human apes into human beings, there was no first human being.  Instead, there appear to have been a continuum of evolutionary forms in a process during which no child was significantly different from its parents. The scientific implication is that some "things" might be in-between non-human and human.  An alternative suggested by Pope John Paul II is that while physical evolution was continuous, spiritual evolution was not. The Pope specifically ruled out an appeal to science when he described this idea.

I would like to end this response with the telling of an episode that occurred while I was hiking through the jungle with my two sons and my wife in West Africa last summer.   At one point, my son walked past what looked like an eight-foot long yellow-greenish tree tree branch that had fallen onto the path.  But the "branch" suddenly became animated and slithered off into the undergrowth. It was a snake of the Green Mamba species, our guide told us.   "Is it dangerous?," I asked.  "It depends," he said. "If the snake is able to deposit part of its spirit into the person, it will consume the person's spirit and death will surely follow.  However, if the snake's spirit doesn't make it across in the bite, the person will suffer no permanent harm."

This was not a parable, it was cultural knowledge -- an attempt by uneducated people to make sense out of the otherwise unexplainable observation that some people died while others recovered completely after a Green Mamba bite.  I didn't believe it, but I couldn't disprove it.  It failed to provide any predictions -- it was not falsifiable and, therefore, it was not scientific.

Professor Lee and George have argued that embryos with wrong numbers of chromosomes are not really embryos because they do not have the capacity to develop into mature human beings.  This is true for 90% of human embryos which have an extra copy of chromosome 21 (in particular), but 10% do develop into human beings born with Down Syndrome.  In these cases, according to Lee and George, we can tell in retrospect that the embryo must have been a human being as well.

Lee M. Silver Professor of Molecular Biology and Public Policy Princeton University