Physicalism and functionalism, both within and outside of Christian thought, attempt to define personhood by equating the concept with a set of neurological and/or cognitive criteria. While these approaches are often fueled by a desire to identify those traits that are uniquely human, by necessity they often remove the label of “person” from the developmentally disabled or those who have suffered traumatic brain injury. This paper will address and explore these issues and propose that a logical way of countering such physicalist and/or functionalist conclusions is to affirm the ontological primacy of the person as a living member of the human species, thereby affirming as well the reality of human nature in Aristotelian and Thomist lines of thought. The human central nervous system will be addressed within the context of the potentialities of the human being, not as a thing which needs to reach a stage (or actualization) of development in order to make an organism distinctively human. The paper will suggest that physicalist and/or functionalist approaches to human cognition, behavior, and neuropsychology are far less problematic ethically if they affirm the existence of human nature prior to examining function.