I just finished Robert Spaemann's wonderful book: Persons: the difference between someone and something. It is a difficult read, but worth it, as his argument that persons are constituted by their membership in humanity is one of the best answers I can find to the arguments of utilitarians such as Peter Singer. There is a lot at stake here, but my favorite part of the book was the last chapter, when he explained how the severely disabled give more to human society than they require from it. They do this by being an "acid test" for our humanity--they force society to think of persons as more than the sum total of their properties.
“In fact, however, they give more than they get. They receive help at the level of sustaining life. But for the hale and hearty portion of mankind giving this help is of fundamental importance. It brings to light the deepest meaning of a community of persons. Love or recognition directed to a human being is not, as we have seen, directed merely to personal properties, though it is the personal properties that allow us to grasp that a person is there. Friendship and erotic love develop mainly in response to the beloved’s individual personal properties. A disabled person may lack such properties, and it is by lacking them that they constitute the paradigm for a human community of recognizing selves, rather than simply valuing useful or attractive properties. They evoke the best in human beings; they evoke the true ground of human self-respect. So what they give to humanity in this way by the demands they make upon it is more than what they receive.” (244)
