Some people may think that what makes killing the innocent is in part that the prospective victim has interests, and what in part grounds the prospective victim's having interests is her having desires. This makes for a neat argument against the claim that the prohibition extends to embryonic and early fetal humans, since these appear not to have desires.
But I think the position leads to absurdity. Suppose Martha is in a severe but temporary depressive episode such that she has only one desire: to die. In such a case, killing Martha is murder. (Some defenders of euthanasia may disagree in the case where the depression is permanent, but what I said is uncontroversial. If need be, add that Martha does not consent.) According to the above story:
- Martha's one and only desire is to die.
- That killing Martha is wrong[note 1] is partly grounded in the fact that she has interests.
- That Martha has interests is partly grounded in the fact that she has desires.
- If p is partly grounded in q and q is partly grounded in r, then p is partly grounded in r. (Transitivity)
- That ∃x(Fx) is partly grounded in the fact that Fa, where a is some particular such that Fa. (Weak form of Existential Grounding)
- That Martha has desires is grounded in the fact that she has a desire to die. (1 and 5)
- That killing Matha is wrong is partly grounded in the fact that she has a desire to die. (2, 3, 6 and 4)
Now, the case of someone whose only desire is to die may seem odd, and maybe someone could say that in odd cases one gets odd conclusions like (7). I don't know that that would be a satisfactory answer. But let's give a less odd case. Marcus desires to die, but he also has a full suite of other desires. I shall assume the case is to be elaborated in such a way that to kill him would be murder. To run the argument in the case of Marcus we need this principle like (5):
- That ∃x(Fx) is partly grounded in the fact that Fa, for any particular a such that Fa and such that the fact that Fa is not itself partly grounded in ∃x(Fx).
- That Marcus desires to die is not itself partly grounded in the fact that he has desires.
- That it is wrong to kill Marcus is partly grounded in the fact that Marcus desires to die.
[Edited. The first version attributed the view that right to life is grounded in having desires to Boonin. But I think that attribution may be incorrect--he may only be claiming that it is grounded in (ideally, dispositionally) having a desire for life. See here.]