The Value of a Life
It is natural to think that the value of a life depends on the nature of that life. The value of a life is internal to it. It depends on what happens to the individual, what he or she accomplishes, what he or she is. But this way of thinking depends on an assumption of uniqueness—to the effect that the individual has no duplicate. Suppose instead that you have a precise duplicate living somewhere, an individual who is exactly like you physically and mentally. If you accomplish something, so does your duplicate; there is nothing you can do or feel or think that your duplicate cannot do or feel or think. If you die tomorrow your duplicate will carry on, living a life exactly like yours. What if you have a million such duplicates dotted all over the universe? There is no shortage of people sharing whatever it is that characterizes your life: tokens of your type exist in abundance. When you die no one will be able to say, “They broke the mold with that one!” Whatever you were doing one of your duplicates could step in and do it. There is nothing unique about you, nothing irreplaceable, nothing special—people like you are a dime a dozen. Given suitable arrangements, no one would even notice your absence, not even your spouse.
But what about your value to yourself: does that also depend on the contingent fact of uniqueness? I fear so: it will be reduced by the existence of duplicates– you will value your life less than you would if you had no duplicates. For you will see that there is nothing uniquely valuable, objectively speaking, about your individual life, however valuable it may be as a type.  You may not want to die considered as an individual, but you have to admit that you are not contributing anything that no one else can contribute. Part of the value of your actual life is what you uniquely are, but if we abolish uniqueness that part is no more. Uniqueness contributes to the value of a person’s life, but uniqueness is not a necessary truth. In some possible worlds you are as common as dirt; in those worlds your life is less valuable than it is in the actual world.
We must conclude, then, that the value of a life depends, partly at least, on facts that lie outside the confines of that life—on who else exists. Compare species: it is worse if a qualitatively unique species goes extinct than it is if a species with a biological duplicate goes extinct. Suppose two distinct species of birds converge on the same phenotype after millions of years of evolution from a remote common ancestor: the intermediate species go through a different evolutionary history, but end up in the same place. We would say that they are distinct species, but they are qualitatively indistinguishable. If one goes extinct, the other remains. Surely this is less bad than a qualitatively unique species going extinct. Uniqueness matters to value. Similarly, the value of a human life depends on what other lives there are; it is not entirely internal to the individual life. The value of your individual life can be undermined by the existence of people just like you. It is less of a tragedy if you die in a world in which there are others similar to you.