We set forth and defend a natural law account of the fundamental dignity of persons. The basis of fundamental dignity--and of the possession of fundamental rights--is being a person: a being with a rational nature. What distinguishes human beings from other animals, what makes human beings persons rather than things, is their rational nature, that is, their having a nature oriented towards enabling them to shape their lives by their deliberate (rational) and free choices. One’s dignity and status as a person derives from the kind of substantial entity one is, namely, a human being, and, as such a creature whose nature is a rational nature. Because personhood is based on the kind of being one is – a substantial entity whose nature is a rational nature – one cannot lose one’s fundamental personal dignity as long as one exists. Although not all persons need be human beings (if, say, there are angels or intelligent Martians, they too are persons), all persons have a rational nature. And so every human being, from his or her coming to be until he or she ceases to be, is a person, and as such a bearer of inherent dignity and fundamental rights.