"We shall adopt an analogous formula, with the reservation that feelings and cognitive configurations do not depend solely on the existing "field," b… - Jean Piaget

In this way things, external objects, are assimilated to more or less ordered motor schemas, and in this continuous assimilation of objects the child's own activity is the starting point of play. Not only this, but when to pure movement are added language and imagination, the assimilation is strengthened, and wherever the mind feels no actual need for accommodating itself to reality, its natural tendency will be to distort the objects that surround it in accordance with its desires or its fantasy, in short to use them for its satisfaction. Such is the intellectual egocentrism that characterizes the earliest form of child thought.

There are no really solipsistic philosophers, and those who think they are deceive themselves. The true solipsist feels at one with the universe, and so very identical to it that he does not even feel the need for two terms. The true solipsist projects all his states of mind onto things. The true solipsist is entirely alone in the world, that is, he has no notion of anything exterior to himself. In other words the true solipsist has no idea of self. There is no self: there is the world. It is in this sense it is reasonable to call a baby a solipsist: the feelings and desires of a baby know no limits since they are a part of everything he sees, touches, and perceives. Babies are, then, obviously narcissistic, but not in the way adults are, not even Spinoza's God, and I am a little afraid that Freud sometimes forgets that the narcissistic baby has no sense of self. Given this definition of solipsism, egocentrism in children clearly appears to be a simple continuation of solipsism in infants.. Egocentrism, as we have seen, is not an intentional or even a conscious process. A child has no idea that he is egocentric. He believes everybody thinks the way he does, and this false universality is due simply to an absence of the sense of limits on his individuality. In this light, egocentrism and solipsism are quite comparable: both stem from the absence or the weakness of the sense of self.

Every psychological explanation comes sooner or later to lean either on biology or on logic (or on sociology, but this in turn leads to the same alternatives).