Say that an entity has "very high value" provided that it is wrong to destroy that entity for the sake of any good less than that of the saving of a typical life of a human person. (This is rough and heuristic, because incommensurability is very problematic here.) Human persons, then, have very high value. If fetuses have very high value as well, then it follows that typical abortions are wrong, but it does not follow that abortions done to save the mother's life are wrong. (My view is that they are wrong, because fetuses not only have very high value but are deserving of the same moral respect as adults. But I shan't be defending that here.)
One might try to argue that fetuses lack very high value on the basis of the following argument:
- Only things that exhibit P have very high value.
- Fetuses lack P.
- Therefore, fetuses lack very high value.
But with these ways of filling in P, the argument is unsound because (1) is false for such P. Here is one example (taking "things" widely to include mereological sums or other collectives): the genus Equus, which currently comprises horses, donkeys and zebras. No member of this genus has P, and the genus as a whole also lacks P. But it would be wrong to wipe out the genus for the sake of any good less than that of saving a typical human life. My evidence for this is that a lot of people will think that it would not be irrational to sacrifice one's life to save the genus from extiction, and that it would be problematic even to save one's life at the expense of the genus. Where something less than a human life is at stake, it seems that it would be wrong to destroy the genus.
If this example does not convince, let's up the numbers. Suppose there is a galaxy, containing a hundred thousand planets teeming with life. The intellectual level of the life in that galaxy does not individually exceed that of a horse, and there is no collective intelligence, either. Then that galaxy, with all its living contents, does not exhibit P. But, surely, it has very high value—it would be wrong to destroy it, with all its contents, for any good that is less than that of the saving of a typical human life. And it is not implausible that it would be wrong to engage in such vast destruction of animal life even to save a typical human life.