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The term “Judeo-Christian” has entered our civic vocabulary for good reason. On many of the deepest issues of human life”the meaning of sex, the dignity of the family, the creation of human beings”Jews and Christians stand together against the secular image of man.

But occasionally, even close friends have disagreements. In a March 2005 essay in the online magazine Slate , William Saletan observed that traditional Catholics and conservative Jews do not always think alike when they gather at meetings like those of the President’s Council on Bioethics. According to Saletan, Catholics raise deep questions and then presume to answer them with divinely confident reason. Jews raise those same deep questions but seem less certain that reason can ever finally settle them. Catholics oppose clear evils like embryo destruction. Jews worry about diffuse evils like the “corruption of our sensibilities.”

There’s some truth in Saletan’s claim, though matters are, of course, much more complicated. The particular Jews he discusses”Leon Kass, Charles Krauthammer, Yuval Levin, and even me”are hardly representative of Jewish bioethics. In many respects, we are outcasts. We oppose most or all forms of embryo research, for instance, and vehemently oppose the creation of embryos solely for research and destruction. By contrast, with all the division among the branches of Judaism”about keeping Kosher, intermarriage, driving on the Sabbath”destroying embryos for research is a point of remarkable theological agreement. The preeminent Reform, Conservative, and Orthodox Jewish organizations in America have all given their ethical endorsement, seeing embryonic stem-cell research as not only permissible under Jewish law but an embodiment of Jewish values. Reverence for life means seeking cures for disease; ex vivo embryos are a justified sacrifice”or little sacrifice at all”in the sacred cause of medicine.

A few prominent Jewish ethicists and halakhic experts dissent, seeing embryo destruction as potentially a prohibited form of feticide. But these voices are in the Jewish minority. Most Jewish thinkers support embryo research with few qualms, and many Jews see opposition to embryo research”or even the denial of federal funding for such research”as an illegitimate imposition of Christian values.

Rabbi Elliot Dorff is a typical example. His guidelines on embryonic stem-cell research”adopted nearly unanimously by Conservative Judaism’s Committee on Jewish Law and Standards”begin by describing, at great length, the cutting edge of stem-cell science: the various methods and sources for deriving embryonic stem cells, the potential to test new drugs and develop new cellular therapies, and the state of research at different American laboratories. The document revels in its scientific sophistication before turning to the fundamental ethical question: Should Jews support the destruction of human embryos for research?

To answer this, Dorff turns to Jewish law on abortion, and especially the Jewish understanding of what embryos and fetuses are as they develop. After forty days, he says, the fetus is classified by the ancient rabbis as “the thigh of its mother”; before forty days, he says, the embryo is “simply water.” Dorff says that it makes sense to follow such teachings only if they cohere with the truths of modern science. And then, inexplicably, he concludes that they do, ignoring the significance of what we now know biologically: that a new organism exists from the moment of conception; that the very first cell divisions are orderly and purposeful; that forty days is a meaningless moment from the standpoint of continuous embryological development; that by forty days the primordial head, arms, and legs have already formed, the primitive heart tube is present, the nerves of the face are developing. The notion that “simply water” is the best metaphor for understanding the unfolding human being in our care is absurd. It is morally and theologically irresponsible to seek the fruits of modern science in the form of stem-cell research without confronting the facts of modern embryology in order to understand what embryos really are.

Jewish thinkers such as Rabbi Dorff commit two errors simultaneously. They embrace modern biomedical science as a faith in itself, and thus lose the mystical vision that might allow them to see embryos as more than sim ply microscopic cells. And they appeal”oft en se lectively”to ancient religious sources without confronting the new scientific facts that make some of these sources a problem. They are, at once, too attached to modern biology and too removed from modern biology. And one wonders whether some of Dorff’s confusions”like describing gametes and embryos in the same breath as “potential building blocks of life,” or equating embryos with other cells of the body, or describing the destruction of an embryo as “taking a part of an object””are not deliberate efforts to make embryo research seem more innocent than it is.

Perhaps the problem is simply that most Jewish thinkers have chosen one Jewish value”the good of healing”as the prism through which to see the old sources. Other considerations”like the law against deliberate killing or the belief in the sanctity of every life as created in the image of God”might lead to different conclusions. The Jewish sources themselves pull in many directions: the Zohar declares that “he who causes the fetus to be destroyed in the womb.. . destroys the artifice of the Holy One . . . . For these abominations the Spirit of Holiness weeps.” R. Meir Simchah says that the killing of a fetus is punishable by “death at the hands of heaven.” Nachmanides finds that the Sabbath may be violated to save an unborn child, even in the first forty days of development. Sanhedrin 57b interprets the biblical text “Whoso sheddeth the blood of man, by man shall his blood be shed” to include “Whoso sheddeth the blood of man, within man shall his blood be shed.” And who is a “man within man,” the rabbis ask? “A fetus within the womb.”

To be sure, there are many Jewish sources that cut in other directions, especially in the standing they accord to embryos in the first forty days. But these sources deal mostly with the laws of purity for potentially pregnant women or for women who miscarry; they do not deal directly with the moral meaning of deliberately killing early embryos. Likewise, the key text in Exodus, which requires a man who causes a miscarriage by colliding with a pregnant woman to pay a monetary fine, does not deal with the meaning of deliberate killing. Deliberate killing, however, is what embryo research necessarily requires, especially research that creates embryos solely for exploitation and destruction.

Some Jewish thinkers, including Dorff, argue that the embryo ex vivo has limited moral standing be cause it cannot develop to term outside the womb. But surely all human beings deprived of the environment they need to flourish have “limited potential for life.” A bird trapped in a cage may never learn to fly, but it is no less a bird for the harm we caused by putting it there. A grown woman without food or water will surely die, but this lack of sustenance does not make the doomed person less than human. If anything, it challenges the humanity of those who left her there to die in the first place.

Perhaps the one great halakhic exception to the pro-embryo research consensus in modern Judaism is Rabbi J. David Bleich, a giant of ethical and legal scholarship in the Orthodox Jewish world. Bleich rejects the argument that embryos and fetuses in utero possess greater moral standing than embryos and fetuses ex utero . But he reminds Jews to stand more humbly before the mystery of new life by reminding them of Ecclesiastes: “As thou knowest not what is the way of the wind, nor how the bones do grow in the womb of her that is with child; Even so thou knowest not the work of God Who doeth all things.” And Bleich concludes his meditation on the ethics of stem-cell research by praising the Catholic Church for its witness in defense of nascent human life: “The Catholic Church now uniquely fulfills a different role in the transcendental divine plan, i.e. , it tenaciously promulgates the notion of the sanctity of fetal life and the teaching that abortion constitutes homicide. Non-Jews who engage in that endeavor do so with divine approbation. Non-Jews engaged in fulfilling a sacred mission are surely deserving of commendation, applause, and support.”

Bleich is not convinced, as Catholics are, that early embryos are the moral equivalent of full human persons. Neither are some of the Jewish conservatives involved with the President’s Council on Bioethics. Bleich’s questions are grounded in the mystery of Jewish sources, and the chairman of the council, Leon Kass, sees a possible tension between our moral intuitions about early embryos and the rational account of early embryos as full persons; he opposes embryo destruction but is not convinced that ex vivo embryos are necessarily equal. My own view is that the Catholic arguments are indeed the most rational, but accepting them in a moment of trial”such as choosing between the child who is dying and the embryo who might save him”would require a faith that is truly other-worldly and thus seemingly absurd to this-worldly eyes.

But whatever fine philosophical differences may exist in theory, Jewish conservatives who engage publicly on these issues have spent the past several years fighting for prohibitions on embryo destruction. As Jews, we know well what it means to treat some human lives as less than human, or some human beings as there for experimentation. We know the moral hazards of justifying such dehumanizing violations on the grounds that embryos are “going to die anyway,” just the way some Nazi doctors justified their inhuman experiments. Embryo destruction is not the moral equivalent of the Holocaust, but the lessons of the Holocaust should give us the wisdom to oppose making embryo destruction the new foundation of modern medicine. That, it seems to me, is the heart of Jewish wisdom.

But it is also only part of the story. In the post-Holocaust age, the Jewish mind is not only keenly aware of the dangers of mistreating innocent life. Jews are also afraid of the demographic death of the Jewish people. The pathos of infertility”a continual theme in the Hebrew Bible”is more powerful than ever, while the place of procreation remains central to the Jewish idea of holiness, to Jewish self-understanding as a sacred people, to the Jewish obligation of passing down God’s way from one generation to the next. As it says in Yevamot :

“Should the number of Israelites happen to be two thousand and two myriads less one, and any particular person has not engaged in the propagation of the race, does he not thereby cause the Divine Presence to depart from Israel?” Abba Hanan said in the name of Rabbi Eliezar: “He deserves the penalty of death; for it is said, ‘And they had no children, but if they had children they would not have died.’” Others say: “He causes the Divine Presence to depart from Israel; for it is said: ‘To be a God unto thee and to thy seed after thee’; where there exists ‘seed after thee’, the Divine Presence dwells among them; but where no ‘seed after thee’ exists, among whom should it dwell? Among the trees or among the stones?”

With this in mind, think about how human embryos came to exist outside the body at all, to be seen with human eyes and held with human hands. We produced embryos outside the protective darkness of the womb in order to give those whom nature made barren a natural child. In vitro fertilization was a technological answer to Sarah’s laughter and Hannah’s cry: “O Lord of Hosts, if You will look upon the suffering of Your maidservant and will remember me and not forget Your maidservant, and if You will grant Your maidservant a male child, I will dedicate him to the Lord for all the days of his life.” To give birth is to be eternally remembered; to be childless is to be eternally forgotten.

Many orthodox Jews see this as grounds for defending even reproductive cloning in certain situations if it were possible and safe. As Yitzchok Breitowitz, one of Orthodox Judaism’s finest ethicists and halakhic experts, has argued: “Let us assume that the individual is the last survivor of a family that was decimated in the Holocaust and let us assume that he was castrated in a concentration camp. If he dies, there will be no perpetuation of his family line . . . .Cloning is as close as the couple in this scenario could possibly come to producing a child that is on some level the genetic product of both of them. One clearly could defend, I think, the morality of the use of reproductive cloning for that limited purpose.” To some degree, every Jew after the Holocaust feels like a Holocaust survivor. And while Breitowitz explores the many dilemmas raised by reproductive cloning, he believes that cloning to produce children presents no inherent problem from the standpoint of Jewish ethics and Jewish law. The wisdom of engaging in human cloning should be judged case by case, he says, and surely not banned by the state.

And so here we have yet another great Jewish irony and internal Jewish conflict: For decades Leon Kass has been the most eloquent and passionate opponent of human reproductive cloning, seeing it as a violation of the dignity of human procreation, a violation of the relationship between the generations, a violation of the uniqueness of new human life, a violation of the sexual character of human reproduction. And yet, it is Orthodox Jews who make the ethical case for cloning with greatest force”as a way to perpetuate a holy people when it is the only biological alternative.

The Jewish defense of cloning strikes me as woefully misguided”a deep misunderstanding of what it means to participate as husband and wife in the creation of new life. But the Jewish defense of cloning is a perversion of something more genuine: the special meaning of procreation within Judaism, and what it means not only for the human family in general but for this particular human family. As Kass himself has explained, children are part of the Jewish answer to mortality, a Jewish way of participating in the immortal. Children connect Jews directly to Abraham, Isaac, and Jacob”to those blood relatives chosen by God to bring God’s new way into the world. In prayer, Jews sing l’dor v’dor : “from generation to generation.” In law, Judaism passes down through birth, not baptism. This is why in vitro fertilization finds nearly universal support within all branches of Judaism”as permissible, never obligatory”and why even those Jews who oppose embryo research often reluctantly embrace in vitro fertilization.

Catholics, in their more universalistic wisdom, do not. They oppose in vitro fertilization precisely because it corrupts rather than fulfills the dignity of human procreation; because it separates the unitive and procreative purposes of marriage; because it turns the mysterious birth of new life into a technological project; because it paves the way for the age of human cloning and genetic engineering; and because it destroys thousands of embryos as “byproducts” and abandons thousands more as “spares.” Most deeply, the practice of in vitro fertilization compromises the connection between sex and holiness: the way the sexual encounter of man and wife, created in the image of God, gives them a glimpse of the divine communion of the Triune God and the mysteries of Creation itself.

The good that in vitro fertilization has produced”the many lives now living”is undeniable. But so are the moral hazards. As a universal ethic, the Catholic position is compelling, certainly in its prudence about the many evils in vitro fertilization has already caused (like mass embryo destruction) or will likely facilitate in the future (like growing genetic manipulation of our offspring).

But as a particularistic ethics”as a practice engaged in by God’s Chosen People, confronted with the suffering of Sarah and Hannah” in vitro fertilization may have a theological purpose and thus a moral justification, if done within marriage and without producing excess embryos. It may be that Jews and Catholics”who share so much in their understanding of the dignity of procreation and marriage”must part ways in their understanding of what holiness in action requires in certain tragic cases of infertility.

As a Jew, I respect the Catholic position deeply and tremble at the practice of initiating new life in the laboratory, even as I wonder at the magnificence of giving new life”flesh of the couple’s flesh”when in vitro fertilization is the only way to do so. And I hope that Catholics tremble when they tell an infertile couple”including an infertile Jewish couple”that having a biological child should not be done even if it could be done, even as they wonder at the magnificence of the Catholic vision of human sexuality and its connection to the mystery of God’s inner life. This is a disagreement among friends mutually devoted to holiness; it is a disagreement that God will surely settle in His own way and in His own time.

But on most things that count”including embryo research”faithful Jews should stand alongside their Catholic friends as Judeo-Christians, opposing together the imageless image of man that secularism offers. I only hope that my Jewish friends, for Jewish reasons, will become more reasonable than they sometimes are.

Eric Cohen is editor of the New Atlantis and resident scholar at the Ethics and Public Policy Center.