Personhood is a legal term not a scientific one. A "person" is who or what the law says he/she/it is. We have a large population of human beings who cannot use reason or logic, Ranging from young children to insane people to adults with dementia but who are persons, with all the due process protections the constitution provides.
I view personhood as something more than a legal definition. Legal definitions change, and sometimes with it, the declared nature of the person. To be a person is to is to be a proper subject of absolute regard. If we accept the Biblical revelation that man is the
imago Dei, the image of God, then
every human being is a person, by nature a thing different from any other, a begin whose very existence is a kind of sacrament. Trying to understand man without recognizing him as the
imago Dei is to miss the point of personhood. Without an explanation, we may try to hold fast to our knowledge of the evil of murdering my neighbor. If we fail to understand the
imago Dei and adhere to the contemporary secular ethics, the ruling tendency is to concede that there are such things as persons, but to define them in terms of their functions or capacities-not to define them in terms of the image of God, but what they can do. Ethicists of today often define "personhood" as the capacity to communicate and conceptual self-awareness. If you can't, then you are not a person. This then opens up a panoplea of what the keepers of the culture will decide dependent upon the vissitudes of the moment. By contrast if we are a person by
nature, then I am a rights bearer,
by nature-not because of what I can do but because of what I am. In short, a person is by nature someone whom it is wrong to view merely functionally an therefore wrong to value merely as a means to the ends or the interests of others. If I am a person because others regard me as a person only because I am able to exercise certain capacities that interest you, that opinion can change with the appointment of a single supreme court justice, a man. Germans referred to this as
lebensunwerten Leben, life, unworthy of life.
The foundational principles behind this, is that we make up moral principles. They are not laws like mathematics, but a derivative of of culture, sort of like the current style of architecture. Without the common moral ground applicable to all persons we are left to the vasile thoughts of men. Disbelief in common moral ground is rapidly becoming a pillar of middle-class prejudice. How often have we heard 'I will not allow you to impose your morality on anyone else'. We see it every day with law suits by atheists to disallow Christians in the military from having access to a Bible.
I think we are not so far apart in the question before us. My problem is that due process, today, seems to be arbitrarily applied.
To quote Forrest Gump, "I'm kinda tired now, I think I'll go home."