The fundamental question I consider is the following: What is it that makes one thing a person and another thing not? I do not provide a complete answer; rather I begin to develop a framework for answering the question. In this essay I do the following: (1) distinguish between the powers possessed by persons and the constitutions of persons, and propose some metaphysical conjectures concerning the relationship between persons' powers and their constitutions; (2) propose for Christians, as well as for (...) others, an hylomorphic soul-body alternative to Cartesianism; (3) highlight some prominent differences between the nature of human persons and the natures of brute animals; and (4) apply this framework for understanding human persons to problems in biomedical ethics concerning the ontological and moral status of human embryos and of the comatose. (shrink)