Should scientists seeking to cure human diseases be permitted to use stem cells from human embryos in their research? Proponents of embryonic stem cell (ESC) research emphasize that it may help in finding cures for diabetes, Parkinson’s disease, heart disease, Lou Gehrig’s disease, and other devastating disabilities and illnesses. Critics acknowledge the possible medical benefits but point out that ESC research destroys embryos. Such destruction, they say, shows insufficient respect for the embryo and, more broadly, insufficient regard for the value of human life. Human embryos, the critics argue, are morally important, and that importance imposes substantial limits on permissible research.

Last summer, President Bush came down close to the critics. He announced that taxpayer dollars could be used only to finance ESC research on stem-cell lines that had already been extracted from human embryos; federal money could not be used, he said, to “sanction or encourage further destruction of human embryos.” Interpretation of this policy has softened recently, making it easier for scientists who accept federal funds to use ESCs in their research, but the debate has grown in intensity because of connections between the use of ESCs and the controversial issue of human cloning. Cloning is one possible source of embryos. And with a bill before the U.S. Senate that seeks to prohibit human cloning for all purposes, including biomedical research, and a majority of the President’s Council on Bioethics recently recommending a four-year moratorium on all cloning of human embryos,1 we urgently need to assess the permissibility of using ESCs.

The moral problems with ESC research have been exaggerated, I believe. But to answer the critics, it is not enough to show that many lives may well be saved and much suffering avoided by new breakthroughs from ESC-based research. Critics acknowledge those benefits—although sometimes with considerable hesitation about their likelihood—but rightly deny that the magnitude of the benefits suffices to justify the research. After all, experimentation on infants is impermissible even if it generates socially valuable results. To respond, then, we need to address the moral criticism head-on, either by showing that human embryos are devoid of moral importance—like a human fingernail or an appendix or a small clump of human cells—or that the kind of moral importance they have is consistent with using them in biomedical research.

The idea that human embryos have no moral importance at all strikes me as wrong, so my case for the permissibility of ESC research assumes that embryos are morally important. I will, however, challenge the conception of that importance endorsed by the majority on the President’s Council in their report on cloning and in an earlier government commission report on stem-cell research.2 

The Sources of ESCs

When a human sperm fertilizes a human ovum, a single cell is created with the potential to grow into a human person. A few days after fertilization, a blastocyst develops, comprising an outer layer of cells that forms the placenta and other tissues needed for the fetus to develop and a hollow sphere that contains an inner cell mass. Cells in the inner mass are called “stem cells,” and they can go on to form nearly all tissues and specialized cells in the human body (e.g., organs and blood cells). Because of this unusual potential, stem cells—sometimes called pluripotential cells—may be useful in treating many illnesses. From this early stage in development until it is nine to ten weeks old, the organism is called an embryo. The embryo passes through a pre-implanted zygote stage, which lasts about seven to fourteen days, and the first eight weeks of gestation. Only after significant cell differentiation has occurred does the organism become known as the fetus. Stem cells can be gathered from the embryo’s inner cell mass; thus originates the term embryonic stem cells.

ESCs can be obtained from three sources: aborted embryos and early fetuses that still have some such cells; embryos generated for in vitro fertilization (IVF) but not implanted; and embryos created by cloning. However the embryos are obtained, they die when the stem cells are removed. A 1999 government report on guidelines for federally funded research involving ESCs acquired through abortion or IVF (that is, not through cloning)3 took the view that while human embryos do not have the moral status of human persons, they should be treated with respect. Treating human embryos with respect entails not using them simply as a means for achieving some further goal. I shall call this the Mere Means Thesis. According to the government report, Mere Means has two important corollaries, one concerning the creation of embryos, the other concerning their destruction.

1. Noncreation: Embryos should not be created for the purpose of conducting research that will destroy them. In particular, embryos should not be created for stem cell research because removing stem cells destroys the embryo. An embryo should only be used in stem cell research if it was created for some other purpose. Otherwise, it is treated as a mere means.

2. Alternate Destruction: Even embryos not created for the purpose of conducting research that will destroy them should not be destroyed in research unless they would have been destroyed in any case. Consider, for example, an embryo left over from an IVF project that will be stored in a freezer. Alternate Destruction says that a researcher should not acquire that embryo and use it to acquire stem cells. That, too, would be to treat it merely as a means and would not show respect.

Together, Noncreation and Alternate Destruction very substantially restrict morally permissible ways to acquire ESCs; they should only be obtained from embryos that were not created for the purpose of being destroyed but that will in any case be destroyed.

To appreciate the force of these restrictions, consider how they apply to the case of cloning as a source of ESCs. Many people assume thatreproductive cloning—that is, cloning that results in a new human person—should be banned, but suppose we clone embryos. If reproductive cloning is wrong, then we have a duty to prevent the cloned embryos from developing into full human beings. So if a scientist clones ten embryos for the purpose of acquiring ESCs but draws ESCs from only five, then the remaining five must not be allowed to survive and grow into cloned human persons. Unless we can freeze the embryos, we will have a duty to destroy any that can develop further.4Development of cloned embryos, however, violates Noncreation, and destroying them violates Alternate Destruction. The result closes off the cloning option altogether.

In a recent New York Times interview,5 Harold T. Shapiro, the chair of the federal panel that produced the original report on the use of ESCs in federally funded research, said that cloning embryos for the purpose of reproduction poses no unique moral problems. Mere Means andNoncreation, however, appear in his panel’s report—theses that conflict with cloning for the purpose of obtaining ESCs for research if, as seems to be the case, destruction of cloned embryos will occur and indeed be required.6

Perhaps, however, Mere Means does not apply to cloned embryos. How might one arrive at that exemption? One reason for thinking that embryos ought not to be treated as means is that the embryo has the potential to develop into a person. Embryos, however, could be created that lack the genetic potential to develop beyond a few days. Some scientists think that using such embryos for research would obviate many moral problems in using ESCs from cloned embryos. Let us call this the No-Potential Solution. In this scenario, Mere Means,Noncreation, and Alternate Destruction do not apply to embryos with such limited genetic potential, even if they apply to embryos with the genetic potential to develop into a person. After all, by destroying an embryo lacking the potential to develop into a human being, we would not be taking away its future because it could have no future.

An alternative way to reopen the option of cloning as a source of ESCs is to say that an embryo’s potential to develop into a human person depends on its environment. Thus Senator Orrin Hatch, an opponent of abortion, came out in favor of ESC research because “life begins in a woman’s womb, not in a petri dish.”7 Hatch’s view seems to be that when an embryo is already in a sustaining environment such as the womb, it has the potential to develop into a person. In a petri dish or a freezer, however, it does not have the potential to develop until someone puts it in a sustaining environment. Hence, even cloned embryos that could develop if put in a sustaining environment do not have the potential to develop when they are not and will not be placed in such an environment. Creating and using embryos in laboratories (as is done in IVF) would create no problem according to this view because they would not have the potential for further development. Interestingly, bioethicist Arthur Caplan—who is no opponent of abortion—also holds this view.8

Notice that in the view proposed by Hatch and Caplan, we achieve theNo-Potential Solution without creating embryos that are genetically unable to develop. If ESCs are taken from embryos deliberately created outside a sustaining environment such as the womb, then Mere Means,Noncreation, and Alternate Destruction may not apply. The fact that an embryo will not develop because we never put it in a sustaining environment is crucial. Achieving the No-Potential Solution in this way would, it might be thought,9 allow us to obtain ESCs from cloned embryos and from leftover embryos generated for IVF.

Because he is pro-choice, Caplan may also believe that when an embryo is aborted, it may be destroyed for its ESCs. Senator Hatch, however, may not share this view, for he thinks that abortion, which fatally interferes with an embryo that is in a sustaining environment, is morally wrong. He may believe it is impermissible to take advantage of an immorally aborted embryo. Therefore, depending on one’s beliefs, the No-Potential Solution may or may not allow us to obtain ESCs from aborted fetuses. 

Problems with Current Policies and Positions

Mere Means and its corollaries impose large restrictions on using ESCs. Unless we endorse some form of the No-Potential view, they appear to close off completely the option of obtaining ESCs from cloning. I want now to offer some reasons for rejecting Mere MeansNoncreation, andAlternate Destruction and for thinking that the No-Potential Solutionis incomplete and even unnecessary. In this section, I will offer some hypothetical cases that suggest that the first three ideas are implausible. In the next section, I will challenge a view that makes the moral importance of embryos depend on their potential to develop into human persons, and I will propose an alternative view of their importance. The upshot is that ESC research is morally much less troubling than much current discussion suggests.

Mere Means. The government report that presents Mere Means seems to be founded on an idea that traces to Immanuel Kant’s moral philosophy. The second formulation of Kant’s categorical imperative says that we should treat rational humanity, “whether in [our] own person or in that of another, always as an end and never as a means only.”10 The embryo is not rational humanity, however, but pre-rational humanity. A pre-rational embryo may have some moral value, but why suppose that the strong Kantian principle applies to it?

To see the force of the question, consider a couple in an IVF clinic. The couple has produced three embryos for implantation and cannot produce any more. The couple hopes for at least two children. Two of the embryos run into trouble, but both could be saved by sustaining them with parts of the third embryo. The third embryo is not in any trouble, is about to be implanted in the womb and would have developed without problems. Still, it seems permissible to use that one embryo to save the other two, even though it is impermissible to kill one person in order to save two people.11 Thus, the couple may use one embryo to save two, but they may not, for example, take organs from one infant child to save two others. If this is so, it is not true that human embryos should never be used as mere means. Embryos have a different moral status than human persons.

Noncreation. According to Noncreation, we must not create embryos we intend to destroy.12 But suppose that a woman dying of heart disease learns that if she becomes pregnant and has a very early abortion that kills the embryo, her body’s reaction to the embryo’s death will prompt a cure for her disease. Would it be morally permissible for her to become pregnant with the aim of aborting the embryo immediately? Noncreation implies that she should not do it, yet it seems permissible.13 To be sure, the example is very odd, but ask yourself what you think. Now suppose instead that the woman could cure her heart disease by carrying her pregnancy to term and having the infant’s heart valves transplanted into her body. In this case, the woman’s conduct is plainly wrong. Once more, moral thought distinguishes embryos from other living human beings—a difference obscured byNoncreation.

Suppose it is permissible for a woman to create an embryo that will be destroyed to cure her heart disease. Why may she not create it in order to give it to a scientist who will destroy it in an attempt to find a cure? Why may she not help create the embryo outside her body, in a laboratory, for the same purpose? If a doctor may help her have an abortion, why may a scientist not help her through ESC research in a laboratory? Perhaps the likelihood of finding a cure is important for assessing the permissibility of these acts, but this should hold whether or not a scientist is involved. Why should it matter that a cure is sought for the very woman whose embryo is donated? Why should she be permitted to help herself but barred from trying to help someone else in the same way?

Another problem with Noncreation is raised by the possibility that one might need to create a spare embryo for IVF in order to use it to keep other embryos alive. (This is an extension of the three-embryo case discussed above.)14 This case reminds us that creating an embryo in order to have a baby does not necessarily mean an embryo must become—or even have a possibility of becoming—a baby. ThoughNoncreation rules out creating an extra embryo for this purpose, it strikes me as morally permissible.

Alternate Destruction. According to Alternate Destruction, we may not destroy an embryo in research unless it would have been destroyed anyway for nonresearch reasons. Suppose, however, a woman is pregnant and discovers early in the pregnancy that she has fatal breast cancer. She has every intention of going through with the pregnancy, as this is her chance to leave a child behind for her family. She is then told that if she aborts the embryo and gives it to a scientist, a drug can be developed that will cure her cancer. According to Alternate Destruction,aborting this embryo is impermissible because the embryo would not otherwise have been destroyed. But it seems permissible for the woman to save her life this way. It seems permissible, too, for her to abort the early embryo to save someone else’s life or to use an embryo in laboratory research even if it would have been implanted had the research not been possible. 

The Moral Importance of Embryos

The basic principles underlying the 1999 government report on federally funded stem cell research—Mere Means, Noncreation, andAlternate Destruction—all seem misguided. These principles are founded on the plausible idea that human embryos are morally important, but they misrepresent that moral importance. How, then,should we understand the moral importance of embryos? I will come at this question a little bit indirectly through a problem raised by cloning.

My criticisms of Mere Means, Noncreation, and Alternate Destructionimply that it is permissible to destroy embryos in more circumstances than if these theses were true. But cloning raises a special problem with these theses. It is widely agreed that allowing a cloned embryo to develop into a human person would be wrong.15 To avoid that wrong, we would have a duty to destroy any cloned embryo that might develop into a human person. Even if it is permissible to destroy an embryo for research purposes, it might be thought wholly objectionable to produce embryos that we subsequently have a duty to kill. An embryo has the potential to develop into a human person and, it might be said, we cannot have a duty to kill an entity with such potential. Hatch and Caplan deny that an embryo in a laboratory has any such potential. Imagine, however, that this embryo has been mistakenly implanted in the womb (or some external gestation device)16—as might happen—and is otherwise fine. Everyone would agree that this cloned and implanted embryo has the potential to develop into a person. Would we nevertheless have a duty to kill it? And is it permissible to start projects that might lead to such mistakes and result in such a duty?

The answer to both questions is “yes” because of the kind of moral importance the embryo has. An embryo is not the sort of entity that can be harmed by the loss of its future. An embryo may have some moral value in the sense that its continued existence, in its own right (even if it is frozen and will never develop into a person), gives us a reason not to destroy it. This value could only be overridden by some good that we can achieve in destroying it, thus ruling out the useless or gratuitous killing of embryos. This is very different from saying that we should not destroy the embryo because that is bad for the embryo.

Consider, by way of analogy, a valuable work of art: say, a painting. A painting is valuable in its own right and therefore should not be wantonly destroyed. But we do not preserve paintings for the sake of the paintings themselves, because their continued existence cannot be good for them. After all, a painting cannot sense, perceive, or experience anything. Likewise, an embryo does not have and never had the capacity to sense, perceive, or experience anything. In contrast, when we refrain from destroying a bird—even if it is less valuable in its own right than a painting—we may be acting for its sake, for it may be good for the birdto continue to exist.17

By not destroying the embryo, can we be acting for its sake because it has the potential to become a human person able to think, perceive, and experience? I do not think so, because even if it is good to be a person and even if there is some sense in which the embryo loses out on becoming a person (from which it is very different), I do not think that the embryo itself is harmed by this loss. I do not think that an embryo is the sort of entity that can benefit from transformation into a person or be harmed by not so transforming. This has something to do with its not being (and never having been) capable of consciousness or sentience,18 and so not capable of being benefited at all, even by turning into the kind of being that can be benefited. Analogously, suppose that a table could, by magic, be made capable of turning into a person. The table is not harmed if it is destroyed instead of being allowed to transform.19 (Harming an entity is not the only way to treat it disrespectfully, of course. For example, overriding a person’s will for his or her own good can be disrespectful. But embryos do not have wills, and so cannot be treated disrespectfully in this way either.)

Notice that the reasons I have given for the permissibility of destroying embryos for research do not yield a principled distinction between embryos in the first two weeks of life and older embryos. Researchers on stem cells intend to use embryos in the first two weeks, before the “primitive streak” appears and marks the first point at which the clump of cells begins to be an individual coordinated embryo. It is possible that other research might find it useful to use older embryos. Some have argued that because an embryo can split before the primitive streak appears and form the bases of identical twins, it does not merit the same protection as the embryo that is the basis for a definite individual person.

I am not convinced this is a morally crucial distinction. Suppose it were possible for children to split into identical twins before age four. A child who will not split still merits protection against destruction. What justifies such protection are the characteristics of the entity. A person has the necessary characteristics, but embryos before or after the primitive streak may not have them. Nor would it be correct to conclude that a child who will split can permissibly be killed on the grounds that the child will soon be replaced by two other people and thus cease to exist.

For these reasons, I do not think that it would be wrong to involve ourselves in a project that would result in a duty to destroy a cloned embryo with the potential to become a human person. I also think that many of my judgments about the permissibility of killing the embryo in the hypothetical cases I explored earlier can be justified by this understanding of the moral importance of a human embryo.

Let us now consider in more detail the question of what has the potential to be a person and whether creating an embryo without the potential to develop into a person is a plausible solution to the many moral issues that surround ESC research. Is it correct to say that an embryo that is not and will not be in a sustaining environment has no more potential for development than an embryo created with a genetic makeup that prohibits development? I do not think so. Consider an embryo that could develop if placed in a sustaining environment but will be frozen instead. Even if it never develops, its genetic capacity for development makes it more valuable in its own right than an embryo without such a capacity. The potential for development into a human person counts for something.

Imagine a magic wand, capable of producing a great effect, that is locked in a museum case and will never be used. Compare it with a nonmagic wand in the same case. Though neither will ever produce any great effects, the former wand has greater intrinsic value in virtue of its potential even though both wands have the same instrumental value. The human embryo that could develop into a human person if it were placed in a sustaining environment is like an unused magic wand.

The difference between embryos with no genetic potential and embryos lacking potential because of their environment can also explain why some antiabortionists object to Hatch’s position. If one believes that the embryo with genetic potential is very important, a possible response is to call for it to be placed in a sustaining environment. This is analogous to how one would treat a child who was in a nonsustaining environment: one would not say that it was permissible to kill the child because the child was in a nonsustaining environment; one would instead try to move the child into a better environment. However, such a position concerning the embryo also implies that frozen leftover embryos from IVF should be adopted and transferred to a sustaining environment at reasonable cost. If this is, in fact, not morally necessary, it is because the value of an embryo with genetic potential does not imply that its potential must be developed or even that it cannot be killed for the sake of an important good. What is most important for the permissibility of using human embryos for biomedical research is not that genetically normal embryos in a nonsustaining environment will not have a chance to develop, but that such embryos need not be placed in a sustaining environment.

Finally, is the creation of human embryos that will die naturally soon after being created a solution to the current controversies? I believe not. The problem here is that we first need to show why embryos can be used in research projects before we can permissibly create entities that are otherwise like human embryos but lack the potential to develop into persons or to live beyond a few days.

To see why, suppose that an embryo already exists with potential to develop, and we seek to take away that genetic potential (without destroying the embryo) in order that we may then destroy it because it lacks genetic potential. Doing this is problematic if we do not first justify our action by showing that embryos are not the sort of entities that have a right to retain their genetic potential or are harmed by having this potential taken away. But if we show these things to be true, we will have gone a long way in proving that it is the sort of entity that can be destroyed.

Now suppose we could create an embryo without genetic potential for continuing life rather than removing such potential. To show that this is permissible, we must first show that it would be permissible to kill the embryo even if it has potential. The following analogy may help. Suppose someone wanted to experiment on human persons but it was objected that this is impermissible because it would lead them to lose the rest of their lives. Creating a human person with a genetic modification that will produce an early death, just so that we could experiment on him without thereby causing him any loss of life, is not a solution, for the sort of entity he would be—a person—would thereby lose out on life, and thus be denied something that is a basic good for him. Hence, it is only permissible to make such a genetic modification to an entity that would not be harmed to a great degree by losing out on more life. If, as I argued earlier, the human embryo is such an entity, then we have already gone a long way in showing that it is the sort of entity that we may destroy even if it has potential for development. To defend the permissibility of creating an embryo without potential, then, we have to defend the very same theses that are crucial to the permissibility of killing an embryo with potential

In conclusion, I want to recall the context of my argument. The discussion of biomedical research using ESCs begins from two basic considerations: first, that such research may have very large benefits; and second, that the research requires the destruction of embryos. Critics argue that we must forgo the benefits of ESC research because destroying embryos fails to show respect for their moral importance.

I have argued that this conclusion is founded on an implausible view of the moral importance of embryos. A proper understanding of that importance must take seriously the fact that the destruction of an embryo is not bad for the embryo. The grave evil that we associate with the destruction of human life—and more broadly with using people as means to an end—reflects the fact that such destruction—and such use—is either bad for the persons whose lives are destroyed<0x00AD> or who are used, or contrary to their will. Embryos, however, have no will, and their destruction is not bad for them. The conclusion is not that we can use human embryos however we want, but that we have no reason to forgo the large benefits that doctors and scientists expect will follow from research on ESCs.20 



1. President’s Council on Bioethics, “Human Cloning and Human Dignity: An Ethical Inquiry,” available on-line under “Reports” at:

2. See National Bioethics Advisory Commission, “Ethical Issues in Human Stem Cell Research, Executive Summary,” September 1999.

3. National Bioethics Advisory Commission, “Ethical Issues.”

4. Charles Krauthammer—a columnist, M.D., and member the President’s Commission on Bioethics—points to this as a decisive reason not to allow cloning for research purposes, even though he agrees that the embryo does not have the same moral status as a person. See his “Crossing Lines: A Secular Argument Against Research Cloning,” The New Republic, 29 April 2002. He also supportsNoncreation, arguing that we must not create human life while intending to destroy it.

5. Howard Markel, “A Conversation with Harold Shapiro: Weighing Medical Ethics for Many Years to Come,&rdquo New York Times, 2 July 2002.

6. Of course, Shapiro may not have agreed personally with the panel’s report, though no dissent was published. Reports by government panels that aim to provide reasons for their conclusions may well be compromises in their conclusions as well as their reasoning. Such reports appear to propose philosophical rationales, but no one on the panel fully endorses the rationale. It is there as window dressing. If this is so, it may not be wise to treat the reasoning in these reports as intended to be correct and so rightly subject to critical examination in the search for truth. On the other hand, such a critical examination is important in order to show that these reports do not embody correct, but only compromise, window-dressing reasoning. I have examined the reasoning provided in other government reports, on organ transplantation and brain death, and also found them wanting. See my “Reflections on the Report of the U.S. Task Force on Organ Transplantation,” The Mount Sinai Journal of Medicine (May 1989): 207-20, and my “Brain Death and Spontaneous Breathing,” Philosophy and Public Affairs 30:3 (Summer 2001). For a philosopher’s discussion of the compromises that are made in serving as advisors to government deliberative panels, see Dan Brock, “Truth or Consequences: The Role of Philosophers in Policymaking,” Ethics 97:4 (July 1987): 786-91; and the last part of D. Green and D. Wikler, “Brain Death and Personal Identity,” Philosophy and Public Affairs, 9:2 (Winter 1980). On some of the issues they raise, see my “The Philosopher as Insider and Outsider,”Journal of Medicine and Philosophy 15 (August 1990).

7. Sheryl Gay Stolberg, “Key Republican Backs Cloning in Research,”New York Times, 1 May 2002.

8. See his discussion of these issues in “Attack of the Anti-Cloners,” The Nation, 17 June 2002).

9. The reasons for the qualification will be clear later: I do not think that the No-Potential solution is really a solution at all.

10. Groundwork of the Metaphysics of Morals, trans. Lewis White Beck, p. 47.

11. On why we should not kill one person in order to save two, see myMorality, Mortality, Vol. 2 (New York: Oxford University Press, 1996).

12. I shall assume in what follows that creating embryos with the intention to use them for research with foresight of the fact that they will certainly die from such use is as contrary to Noncreation as is creating embryos while intending their destruction or creating embryos foreseeing that we will intend their destruction.

13. Notice that those who endorse Noncreation and would support its implication for this case could also think that it is, in general, morally permissible to have abortions. For in most abortions, a woman does not get pregnant in order to have an abortion. Furthermore, one of the reasons given to support the moral permissibility of most abortions is that the embryo (or fetus) is imposing on the woman’s body and its presence is presenting some problem for her. In extreme cases, the embryo may pose a threat to her life, just as heart disease does in the case above. So a person may believe, with no inconsistency, that it is permissible to destroy an embryo that presents a (morally innocent) threat to a woman but that it is impermissible to create and use an embryo (that is not itself presenting a threat to her) simply because destroying it will help the woman avoid another threat, like heart disease.

14. Krauthammer also presents such a case. He thinks that it is analogous to what is involved in cloning and that it is clearly morally impermissible.

15. I shall not here try to contest this assumption that reproductive cloning is wrong, though it can possibly be contested.

16. For discussion of the moral relevance of such external gestation devices for the permissibility of abortion, see F. M. Kamm, Creation and Abortion (New York: Oxford University Press, 1993).

17. Notice that even creatures, such as birds, for whose sake we can act in keeping them alive do not necessarily have a right not to be killed.

18. A being that is capable of sentient experience or consciousness is not one that merely has the capacity to develop into a being that is capable of sentience or consciousness. Also, it is not necessarily a being that has already had sentient experience or consciousness. For example, a being that has never experienced pleasure can still be capable of it, and so it is the sort of being for whose sake I can act in giving it pleasure for the first time and in not depriving it of future life from which it will get pleasure. Here I differ with the view in Bonnie Steinbock, Life Before Birth: The Moral and Legal Status of Embryos and Fetuses (New York: Oxford University Press, 1992), and Mary Ann Warren in Moral Status: Obligations to Persons and Other Living Things (Oxford: Clarendon Press, 1998). They both require that a being have already experienced in order for it to be possible to harm it by killing it.

19. Although it is not a harm to the embryo to lose its potential, it would be an indication of the value of an embryo if we would try to correct a defect in it that interfered with its potential rather than dispose of it and create a new embryo without a defect. Similarly, it can be an indication of the value of a painting if we try to rescue it from damage rather than have an equally good painting created in its stead.

20. I am grateful to Derek Parfit and Jeff McMahan for comments on earlier drafts.