The "Natural Law" Philosophical Basis of Human Rights

To take one example, the traditional understanding of the idea that there is a "natural law" that determines what is objectively right and wrong is inextricably tied to classical realism. For "human nature," as understood by the traditional natural law theorist, is defined in terms of the form that every human being participates in simply by virtue of being a human being. And that means it is something known ultimately and most fully only through the intellect and via philosophical reasoning, not (or at least not entirely or most deeply) through the senses and empirical biology. Moreover, this nature defines certain natural ends and purposes for human beings and their capacities, the realization of which constitutes what is good for them: good objectively, simply by virtue of their participation in the form, and regardless of whether this or that particular human being realizes or (because of intellectual error, habitual vice, psychological or genetic anomaly or whatever) fails to realize it.

To take another, and related, example, a person, being on the view in question a composite of soul (or form) and body (or matter), cannot be identified with either his psychological characteristics alone or his bodily characteristics alone. Moreover, since the soul is just the form of a living human body, for a living human body to exist at all is for it to have a soul, so that there can be no such thing as a living human body -- whether that of a fetus, an infant, a normal human adult or a severely brain damaged adult -- which does not have a soul, and which does not count as a person. For while even a human being who is damaged or not fully formed might not perfectly exhibit the form of the human body (any more than a hastily drawn triangle perfectly manifests the form of triangularity), he nevertheless does exhibit it, otherwise his body wouldn't count as a living human body at all (just as a hastily drawn triangle is still a triangle, however imperfect). One corollary of this is that every single living human body, within the womb or without, severely damaged or not, counts as the body of a person and as a being having all the rights of a person, including the right to life.

--Edward Feser, “The Metaphysics of Conservatism”