The First Year of Life of the Child (1927), "The Egocentrism of the Child and the Solipsism of the Baby", as translated by Howard E. Gruber and J. Jacques Vonèche
Context: 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.
