If we look at Piaget's work, it is clear that the child's sense of self develops out of its relationship with its mother. Children very early begin to identify faces, and begin to recognize familiar faces. The first correlations the child likely puts together are the correlations between the child's sensations (hunger, fear, discomfort, wonder, happiness) and the face of the mother in response. Only much later, when the sensations and the experience of the mother are established in memory, will the child begin to express emotions in language. Not doubt the child may be exposed to a mirror, but no mirror is necessary for the child to develop normally. In some sense, empathy stems from earliest infancy, as the child identifies his or her emotions with the response in the face of the mother.
It is clear that the first and the second person tenses evolve in tandem in the toddler. The child can express their emotions in language, because the child can express these emotions in behavior. The child can empathize with the second person, because they have learned to associate their sensations with the face of the other. In this sense, "I" cannot be understood divorced from "you".
Now the third person is an interesting development. In learning words, the parent teaches the child how to use conventions, perhaps pointing out the names of crayons, and asking the child to name the color of the crayon. In this way, the child picks up a set of authoritative conventions, and once the child has mastered these conventions, the child can begin to attempt to construct descriptions. Note that in learning not just to name, but to describe, the child can struggle even if they more-or-less understand the conventions. Describing does not simply happen automatically, first a child needs to learn how to describe, just as the child needs to learn how to name.
Out of these descriptions, a new tense emerges, the third person, which can be personal (he/she) or impersonal (it). It is important to acknowledge that what we describe is not given in experience. It is only when experience is combined with a certain type of linguistic training that something can be described. Moreover, our descriptions of the world are based on the agreement of the first and the second person. Unless they can agree, there is no true description of anything.
What is necessary for the impersonal world to exist is not the truth of a philosophical metaphysics, be it realism or idealism. What is necessary is an agreement in conventions and an agreement in descriptions. In this sense, the real world drops out of the equation. The so-called "real world" does not have to exist, so long the community agrees on its description of the "real world". In fact, invocation of the "real world" only comes into the equation when there is a dispute between or over descriptions. In terms of Platonism, what we can know is language, and this knowledge is based on interpersonal agreement. If we switch to a more Wittgenstein take, what we cannot doubt is language, and our agreement is the sign of the absence of doubt.
If we consider the case of an apprentice and a carpenter measuring a board before a cut, we can imagine the apprentice measuring and marking the board at the point of the intended cut. We can imagine the carpenter then measuring the board to test the judgment of the apprentice, and either accepting it or correcting it. In this endeavor, the carpenter is on the face of things correct in his or her measurement, by definition. I suppose in some extraordinary situation (say the ruler slips and the apprentice points this out), the authority of the carpenter's measurement can be called into question. Yet the general authority of the carpenter, in most circumstances, cannot be brought into question without ending the apprenticeship.
What is important to understand is that the sensations that both the carpenter and the apprentice experience are private. We cannot see through the eyes of either person, anymore than they can see through our eyes. The agreement is not in the realm of sensation, the agreement is based on what the two persons do and what the two persons say. What unifies people is not an impersonal "real world" but the unity of persons in activity.
In the old days, people spoke of materialism. Today, people speak of naturalism, having given up on developing any cogent definition of what matter is in a quantum world. Naturalism, in its essence, is the claim that first person expressions and second person expressions can be reduced to third person descriptions. For example, the assertion "I am in pain" can be reduced to a third person statement about the physical condition of an organ. Note that this is very different from a correlation between a first person sensation and an experience of a second or third person. Specifically, the correlation is between a sensation and a facial expression. To live in the universe of naturalism is to live in a universe that has no face, that never really smiles or frowns. This is perhaps the attraction of naturalism to some. It is important to note that the face expresses emotions. A face is not an emotion, but the meaning of our emotional concepts cannot be severed from facial expressions. We can--hypothetically--imagine a person who experiences emotions, but who is incapable of expressing them. However, we cannot imagine the human species communicating as it does if everyone lacked this expressive capacity.
Because the face expresses emotions, and emotions cannot be divorced from their bodily expressions, we can see the claim that pain is "really just" a brain state is abject nonsense. There may be a correlation between a brain state and an emotional display, but we are less wrong if we claim that emotions are just a facial expression. I say this because we master the use of emotional concepts in connection with facial expressions, almost never in connection with brain imagery. If human beings didn't have facial expressions, I don't mean to deny that they might not have emotional concepts. However, the meaning of the those concepts would be different, and what is an emotional concept divorced from what it means? Likewise, the idea that emotions are "really just" brain states radically alters the meaning of emotions. This is perhaps the attraction of reductive materialism to some.
What is naturalism as a philosophy really? It is the claim that first and second person expressions can be reduced to third person impersonal statements: "It puts the lotion on its skin." Empirical science no doubt rests on the capacity of persons to agree on third person impersonal descriptions, and to formulate correlations between historic states of systems. Perhaps there is characteristic neural activity, and then facial expressions. But this empirical science forgets what it rests on: a system of linguistic conventions, and practices that are transmitted inter-personally and historically. Without these social conventions, and this social training, the scientific description would be meaningless.
What is the source of the "I"? Grammar. What is the source of the "you"? Grammar. What gives these concepts meaning? Our collective forms of life. If an extraterrestrial observed the brain states of a human being, not knowing our language, and perhaps having different means for expressing affective states, there is no reason why the extraterrestrial would suppose the human being is in pain. After all, we say that ants are in pain because if you shine the sun on them with a magnifying glass they move in the other direction. The reason we say this is there is a correlation between their behavior and our behavior (if you shine the sun through a magnifying glass on our arm, or in our eyes, we jerk away). If no such correlation existed, we would not be able to apply our concept to the ants. Note the imputation is made not on the basis of a physical state of a thing, but rather through the correlations between the behaviors of two different unitary organisms.
Animals are, as we say, self-moving. The organism moves itself, without direct input from the external world. In contrast, a rock will only roll if something rolls it. A person smiles, a brain, a part of a whole, does not. Traditionally, Western Civilization spoke of the soul, or the animate principal, because they observed a unified being. While a symphony requires a conductor, the conductor cannot conduct without the conscious cooperation of the orchestra. The symphony is a harmony that emerges through the cooperative synergy of the many players. Likewise, what the body expresses is not a state of the body's organ, but the expression of a unified being. That is to say, the universe must really have a face, at least in these parts,
It is clear that we cannot understand the meaning of the First Person except through the visible form of the Second Person. It is further clear that the Third Person cannot emerge without the agreement of the First Person and Second Person. Moreover, the Third Person, Impersonal, cannot be understood except as the passive form upon which the First Person, the Second Person, and the Third Person, Personal act in harmony. Because the First Person, the Second Person, and the Third Person are only known through their mutual and harmonious activity, they are ultimately unknowable in themselves, they cannot be separated. Moreover, because they are not divisible, they cannot be understood as one in number. If they were one (as materialism would suppose), then their individuality would collapse. Neither one nor three, but the source of all numbers, all meaning, all persons, all unity, and all description. Our grammar expresses the mystery of Life.