Thanks to comments from Adam, Christian, John and others, I offer this much improved version of Parfit Pills. Assume that the fetus F is identical to the child C or F is not identical to C. A slightly more complicated version of the argument would assume that F and C are two stages of the same 4-D being or they are not. Here is premise (1).
1. (F = C) v ~(F = C)
Let’s assume that the causal consequences of not taking the pills are that three fingers on the right hand of the fetus F become permanently detached (make it four fingers or the entire hand or arm, if nothing less would constitute a harm). Premise (2).
2. M’s failing to take the pill causes the fingers of F to become permanently detached.
Now either M’s actions harm F or they do not.
3. M harms F or M does not harm F.
4. M harms F (Assume)
5. F = C (Assume)
6. M harms C, from 4,5 Leibniz’s Law
So if M’s actions (which causally affect F) constitute a harm to F and F = C, then M harms C. But suppose M’s actions do not harm F.
7. M does not harm F (Assume)
If (5) is also true (viz., F = C) and if it is urged that M does harm C, then we have a violation of Leibniz’s law. If M harms C and C = F, then M harms F.
Finally, suppose that (5) is false. So, we are assuming (8),
8. ~(F = C) (Assume)
If (8) is true then (6) (viz., M harms C) is false. What the mother did caused F to lose his fingers. But F goes out of existence before the distinct being C comes into existence. The being that the mother’s actions affected no longer exists. So though it is true that the mother did cause F to lose his fingers, she did not cause C to lose his fingers. So the mother did not harm C.
Conclusion: The mother caused F to lose his fingers. If, in so doing, the mother harmed C, then she harmed F. In Parfit’s case the mother’s action causally affects F. If, in so doing, the mother harms C, then she harms F. If diminishing F’s future is a harm to F, then taking F’s life is a harm to F.
It might be argued that the mother could at least see to it that C keeps his three fingers. But this is also false. Suppose the mother takes the pill. In that case she causes F to retain the three fingers he would have lost. But, assuming ~(F = C), F goes out of existence before C comes into existence. So though it is true that the mother causes F to retain his fingers, she does not cause C to retain his fingers. C’s fingers were never in jeopardy.