“At the completion of the process of fertilization when the male and female pronuclei of the human progenitors’ sperm and ovum are indistinguishable and lose their nuclear envelopes, the human creature emerges as a whole, genetically distinct, individuated zygotic human organism. This individuated human organism actually has the natural capacity for the person-defining activities of reasoning, willing, desiring, and relating to others.So a blastocyst (a small group of cells inside an outer shell of cells) has the capacity for reasoning, willing, desiring and relating to others? There's no brain, heart or even placenta yet. They lost credibility right there.
No they didn't. You are drawing the line at act, as the 1973 SCOTUS drew the line at viability, while the authors have made the case that potency is criteria enough for knowing that a blastocyst is a human being, who, by its very nature, has intrinsic worth.
If one continues along your line of thinking, the line for establishing when one is a human being can erroneously be drawn anywhere. For example, the age of reason is generally placed at age seven, but a seven year old usually doesn't know how to drive a car. Although seven year olds can't drive cars, they have the potential to learn how. Furthermore, five year olds, generally speaking, have the potential to reason, and drive a car as well. Depending on where the line is drawn (and the agenda of the one drawing it), there can be a justification argued for offing 5 year olds, 7 year olds, babies with only their heads unborn, and anyone else who doesn't meet the established criteria.
Such lines of establishing who is a human being are always subjectively and artificially drawn, and humans are killed if the one with the agenda does it in secret, has a good PR campaign, or has enough power.
At any rate, the true answer lies not in act, or viability, for that matter, but rather in potency.
AR, your problem is with the understanding of the definition of "capacity," and, as LB has said, with the idea that it is necessary for the person to act, or function, at a given level at all or any given moment(s).
Not every member of the species must demonstrate all the qualities of all the members of the species in order to be classified as a member of the species.
Aristotle discussed the "nature" and "power" of beings over 2000 years ago. This is not new philosophy, nor is it solely Judeo-Christian or American philosophy.