K. E. Himma, A Dualist Analysis of Abortion: Personhood and the Concept of Self Qua Experiential Subject, 31 J. MED. ETHICS 48 (2005).
It is usually thought that the issue of whether abortion ought to be legally permitted turns on the moral standing of the fetus. If, according to this familiar argument, the fetus is a moral person, then it has a right to life that, at the very least, defeats the mother's less important right to control her own body. If, on the other hand, the fetus is not a moral person, then the fetus simply does not have sufficient moral standing to defeat a right of any kind; thus, a fortiori, it does not have sufficient moral standing to defeat the mother's right to control her own body. Accordingly, much energy has been focused on trying to articulate the criteria of personhood.
The author argues that, under dualist assumptions, the instantiation of brain activity is a necessary condition for selfhood in the fetus. Thus, the fetus simply cannot be or instantiate a self until brain activity begins to occur. First, according to the ontological thesis, there are two kinds of entity in this world capable of instantiating causally efficacious properties and capable, at least in principle, of existing independently of each other: material substances (bodies) and immaterial substances (minds or souls). Second, according to the composition theses, a human being is a composite of a material substance (that is, a body that has certain biological properties) and an immaterial substance (that is, a mind or soul). Third, according to the self thesis, the soul is identical with the self qua subject: the soul constitutes the inner observer that is experienced by the person as self. Fourth, according to the interaction thesis, material bodies and immaterial minds are capable of causally interacting in both directions. In particular, immaterial minds are capable of causing effects in material bodies, and material bodies are capable of causing effects in immaterial minds.
These four tenants of dualism imply, at the very least, that moral personhood does not begin until the fetus is, so to speak, inhabited by a soul. If persons are conceived as being essentially composites (or unities) of bodies and souls, then a genetically human fetus does not become a person in the relevant moral sense of having a full-blown set of rights until there is a soul that is associated in the right kind of way with the body of the fetus. …