Nicholas Wolterstorff explains the problem with some attempts to ground human rights.
So the problem is this. The classical attempts to account for rights are capacity accounts – capacity for rational agency, capacity to make moral judgments and so forth. But when we come to these severely impaired human beings, they don’t have the capacity to engage in rational agency. They don’t have the capacity to make normative judgments about good and bad, right and wrong, and so forth. So an account which explains why even they have rights has to be an account that does not appeal to capacities, and has to be some sort of … they have to stand in some sort of relationship to something or other whereby they have these rights. And I don’t think there is any plausible answer to whom might they stand in that relationship, other than they stand in that relationship to God by virtue of bearing the image of God and by virtue of being beloved by God.