I suspect that Vlastimil V's (neo-scholastic) understanding of potentiality is similar to the one provided by Matthew Lu in Potentiality Rightly Understood:
The substance view of persons holds that every human being either has the potential to manifest any and all properties essential to personhood or does actually manifest them. For the adherent of the substance view of persons, "potential" does not essentially refer to some possible future state of affairs. Rather, in this conception of what I will call developmental potential, to say that an organism has the potential to manifest some property means that that property belongs essentially to the kind of thing that it is (i.e., is among the essential properties it has by nature). Whether or not a specific individual actualizes the potentialities of its nature is contingent; but those potentialities necessarily belong to its nature in virtue of its membership in a specific natural kind.
I don't understand this. Let the property be rationality. Let organism o belong to the natural kind human being. We assume that man is by nature a rational animal. A human fetus is of course a human being. Suppose the fetus is anencephalic. It too is a human being -- it is not lupine or bovine or a member of any other animal species. But it is a defective human being, one whose defect is so serious that it, that very individual, will never manifest rationality. So how can every human being have "the potential to manifest any and all properties essential to personhood"? That is my question. Now consider the following answers/views.
A1: The anencephalic human fetus does not have the potentiality to manifest rationality. This is because it lacks "the largest part of the brain consisting mainly of the cerebral hemispheres, including the neocortex, which is responsible for cognition." (Wikipedia)
A2: The anencephalic human fetus does have the potentiality to manifest rationality because it is a member of a species or natural kind the normal (non-defective) members of which do have the potentiality in question.
A3: The anencephalic human fetus does have the potentiality to manifest rationality because the natural kind itself has the potentiality to manifest rationality.
I think (A2) is the most charitable reading of the above quoted paragraph considered in the context of Lu's entire paper. Accordingly, a particular anencephalic fetus has the potentiality to manifest rationality because other genetically human members of the same species do have the potentiality in question. This makes no sense to me. But perhaps I am being obtuse, in which case a charitable soul may wish to help me understand. To be perfectly honest, I really would like it to be the case that EVERY "human being either has the potential to manifest any and all properties essential to personhood or does actually manifest them." I would like that to be the case because then I would not have to supplement my Potentiality Argument against abortion with other principles as I have done in other entries.
What's my problem? Let's start with an analogy. It is narrowly logically possible and broadly logically possible that I run a four-minute mile. It is also nomologically possible that I run a four-minute mile. For all the latter means is that the laws of nature pertaining to human anatomy and physiology do not rule out a human being's running a four-minute mile. Since they do not rule out a human being's running that fast, they don't rule out my running that fast.
But note that the laws of human physiology abstract entirely from the particularities and peculiarities of me qua individual animal. They abstract from my particular O2 uptake, the ratio of 'fast twitch' to 'slow twitch' muscle fibers in my legs, and so on. And to be totally clear: it is the concrete flesh-and-blood individual that runs, 'Boston Billy' Rodgers, for example, that very guy, not his form, not his matter, not his nature, not any accident or property or universal or subjective concept or objective concept that pertains to him.
Now consider the question: do I, BV, have the potential to run a four-minute mile? No. Why not? Because of a number of deficiencies, insufficiencies, limitations and whatnot pertaining to the particular critter that I am. The fact that other runners have the potential in question is totally irrelevant. What do their individual potentialities have to do with me? The question, again, is whether I, BV, have/has the potentiality in question. It is also totally irrelevant that the laws of human physiology do not rule out my running a four-minute mile. Again, this is because said laws abstract from the particularities and peculiarities of the concrete individual. Surely it would be a very serious blunder to suppose that the nomological possibility of my running a four-minute mile entails the potentiality of my doing any such thing. That would be a two-fold blunder: (i) potentiality is not possibility, and (ii) potentiality is always the potentiality of some concrete individual or other.
Similarly, the anencephalic individual does not have the potentiality to manifest rationality. The fact that normal human fetuses do have this potentiality is totally irrelevant. What do their individual potentialities have to do with the potentialities or lacks thereof of the anencephalic individual? It is also totally irrelevant that man is by nature a rational animal, that the capacity to reason is 'inscribed' (as a Continental philosopher might say) in his very essence. For the question is precisely whether or not this very anencephalic individual has the potentiality to manifest rationality. My answer, as you may have surmised, is No.
I think I can diagnose the neo-Scholastic error, if error it is. (I hope it is not an error, for then the Potentiality Argument is strengthened and simplified.) Take a look at (A3):
A3. The anencephalic human fetus does have the potentiality to manifest rationality because the natural kind itself has the potentiality to manifest rationality.
This, I submit, is a complete non-starter. Whatever a natural kind is, it itself does not have the potential to be rational. It can no more be rational than humanity in general can run. (I once entered a 10 K event called 'The Human Race.' I did not compete against humanity in general, but against certain particular human critters.)
So it can't be the universal nature humanity that has the potential to be rational. What about the individual or individualized nature, the human nature of Socrates, of Plato, et al.? Could a particular individualized nature be that which has the potential to manifest rationality? No again. For it is but an ontological constituent of a concrete man such as Socrates. It is baby Socrates that has the potential to manifest rationality and excel in dialetic, not one of his ontological constituents. Socrates is more than his individual human nature; there is also the dude's matter (materia signata) to take into consideration. Our man is a hylomorphic compound, and it is this compound in which the potentiality to display rationality is grounded.
My diagnosis of neo-Scholastic error, then, is that neo-Scholastics, being Aristotelians, tend to conflate a primary substance such as Socrates with his individual(ized) nature. Since human nature in general includes the potential to be rational, it is natural to think that every individual(ized) human nature, whether normal or defective, has the potential to be rational. But surely it is not the individual(ized) human nature that has the potential to be rational, but the ontological whole of which the individual(ized) human nature is a proper part. In the case of the anencephalic fetus, this ontological whole includes defective matter that cannot support the development of rationality. Only if one confuses the individual(ized) human nature of the anencephalic individual with the concrete anencephalic individual could one suppose that it too has the potential to manifest rationality.
The fact that Lu's paragraph above is ambiguous as between (A2) and (A3) further supports my contention that there is a confusion here.
My view, then, is (A1). Abortion is a grave moral evil. The Potentiality Argument, however, does not suffice as an argument against every instance of it. This is not to say that the aborting of the anencephalic is morally acceptable. It rather suggests that the PA requires some form of supplementation.