In my graduate program in theology we studied Boethius. I really dislike his definition of "person" as a rational being ("an individual substance of a rational nature"). God isn't rational either, in the sense that He or angels use logic. Yet He is personal. My friend’s son, who is severely disabled, isn't rational. So why is he a person? I know he is. What is meant by the "metaphysics of the person" anyway?
I think you are imagining a rational being as someone who sits around proving theorems. Expunge that image from your imagination! Rationality means being oriented toward truth as such; humans are rational because they want to know what reality means. You probably also think Boethius’ definition is cold, because it says nothing about love. But it does, because only rational beings love. By putting us in connection with what is true and good and beautiful, rationality puts us in touch with the truth and good and beauty of the other person: It makes it possible to say, “It is true and good and lovely that you exist!”
Brute animals don’t care about what is true as such. A dog may be interested in where he can get some food, but he doesn’t ask “What does all this mean?” A brute animal can love in the analogical sense of experiencing affection, but only a rational being can love in the true sense of entering into personal communion, willing all possible truth and goodness and beauty for the other, caring for the other because of what he really is rather than what he gets from him.
Of course God and the angels are rational. God, a unity of three persons in one substance, simply is love; and the minds of God and the angels, like ours, are concerned with truth as such. You point out that God and the angels don’t use logic to know things, but being rational is not the same thing. Thomas Aquinas thought that rational minds of the angelic sort know certain kinds of things by direct insight without having to engage in that sort of inquiry. And the mind of God doesn’t inquire into truth and meaning; it creates truth and meaning.
Yes, your friend’s disabled son is a person! He possesses the same essence you and I do: He is, in his very essence, in the kind of reality that he has, a rational being, not a sub-rational being. This does not mean that the rational powers that he possesses in potentiality will be actualized; something is impeding their development. But whatever impedes them is an impediment – it is alien to what he is by nature.
Functionalists, like Peter Singer, can’t recognize the personhood of people like your friend’s son because they don’t believe in essences, and they don’t distinguish essences from impediments. They think someone is a person only if he isperforming the rational functions. In their view he has to be actually doing things like making plans, carrying them out, and communicating complex messages, or he isn’t a person. So an unborn child, an infant, a toddler, a deaf-mute who has not learned sign language, and a person in a coma are not persons in their view. To be consistent with their premises, they should also say we aren’t persons when we are asleep or knocked out. And they should say that personhood is a matter of degree -- that people who are better at such things as making plans also possess greater degrees of personhood.
By contrast, essentialists, like us, and like Boethius, think someone is a person not because of what he can do but because of what kind of being he is. Humans are beings of a natural kind that possesses rational powers in potentialityeven if these powers are impaired or remain undeveloped. So all human beings are persons, and personhood is not a matter of degree.
To answer your final question: Metaphysics asks “What is there?” Of course geology also asks that question: What kinds of rocks are there? And grocery clerks ask it when they take inventory of their stock: What kinds of items are on the shelves? But metaphysics is different because it asks what sorts of beings there are in general. For example, is matter all there is? What are thoughts and meanings? Are there essences? Does God exist? What kind of thing is beauty – is it a property of things, or is it only a feature of our perceptions?
The metaphysics of persons asks whether there are such beings as persons, and what kinds of beings they are. An individual person is a complete individual reality, existing in itself, different from all other somethings, made for rationality, the ultimate possessor under God of all it is and does. A person is not just a piece or part of something, it is not just an instance or process of something, it is not just a clump of different somethings. Nor is it merely a thing to be owned, a thing to be used, or a thing of any sort at all. It is not just a what, but a who. All of that is implicit in Boethius’ definition.