Ratio Juris 21:2 (June 2008), with Patrick Lee.
We argue that all human beings have a special type of dignity which is the basis for (1) the obligation all of us have not to kill them, (2) the obligation to take their well-being into account when we act, and (3) even the obligation to treat them as we would have them treat us, and indeed, that all human beings are equal in fundamental dignity. We give reasons to oppose the position that only some human beings, because of their possession of certain characteristics in addition to their humanity (for example, an immediately exercisable capacity for self-consciousness, or for rational deliberation), have full moral worth. What distinguishes human beings from other animals, what makes human beings persons rather than things, is their rational nature, and human beings are rational creatures by virtue of possessing natural capacities for conceptual thought, deliberation, and free choice, that is, the natural capacity to shape their own lives.
Wiley Library [pdf]