Since I see no external source of value in the world, I do have to consider value as something assigned by humans rather than existing independent of human opinions. And I see its purpose as to provide a lever for influencing the behaviour of humans (including myself) in such a way as to contributes to a state of affairs that all appropriately enlightened humans consider “good”.
But since I am unable to find a well defined concept of overall benefit to maximize over all possible options according to the utilitarian prescription, I have to assign value on some other less computable basis. (And I also have to accept that others may assign value differently and that my own assignments of even relative value may not be consistent over time.)
Assuming (as I do) that the “value” of something is due to the extent to which expecting people to protect it contributes to a state of affairs that all appropriately enlightened humans consider “good”, it seems to me that there may be some value in a mindless human life because having a knee-jerk protective reaction on the basis of life itself may be useful in contexts where an attempt to compute some kind of aggregate utility would either cause undue delay and/or might even end up in a some kind of infinite loop.
By this I mean that encouraging a respect for all human life – even the mindless kind – may lead to better action in situations of uncertainty (such as when the mindfulness of an individual is not apparent and we have every reason short of absolute certainty to believe that there is no mind involved). Yes, it amounts to relying on ethical “gut reactions” as a heuristic short-cut in moral thought, but I don’t really believe that we have any other option.
Of course even the utilitarian might also see value in protecting mindless human life in order to avoid the distress that abandoning it might cause to mindful beings who have an emotional attachment to that mindless one. My child’s doll may feel no pain itself but it’s preservation has value to the extent that its destruction would cause her pain. Similarly, a fetus whose existence has been advertised may have value as a result of the emotional attachment of complete strangers even if that attachment has no rational basis. We may well judge that value to be negligible in comparison with that of respecting the mother’s wishes but it exists nonetheless.