Intellectually, the period of infancy might be said not really to be over before the age of twenty-five, by which time habits of mind have become fairly well fixed. The brain and the nervous system remain fairly plastic up to that time, and if inquiry and learning have themselves become habitual, plasticity may last even longer. In the cases of the greatest intellects, of a Darwin, or a Newton, one might almost say the period of infancy lasts to old age. To be still learning at sixty is to be still a child in the best sense of the word. It is still to be open rather than rigid, still to be profiting by experience.
The great social advantages of the prolonged period of infancy lie in the fact that there is a unique opportunity both for the acquisition by individuals and for the imposition on the part of society of a large number of habits of great social value. The human being, born into a world where there are many things to be learned both of natural law and human relations, is, as it were, fortunately born ignorant. He has instincts which are pliable enough to be modified into habits, and in consequence socially useful habits can be deliberately inculcated in the immature members of a society by their elders. The whole process of education is a utilization of man's prolonged period of infancy, for the deliberate acquisition of habits. This is all the more important since only by such habit formation during the long period of human infancy can the achievements of civilization be handed down from generation to generation. Art, science, industrial methods, social customs, these are not inherited by the individual as are the instincts of sex, pugnacity, etc. They are preserved only because they can be taught as habits to those beings who come into the world with a plastic equipment of instincts which lend themselves for a long time to modification.
CONSCIOUSNESS OF SELF AND REACTION TO IDEAS. A significant difference between the actions of human beings and those of animals is that human beings are conscious of themselves as agents. They may be said not only to be the only creatures who know what they are doing, but the only ones who realize their individuality in doing it. Dogs and cats are not, so far as we can draw inferences from extended observation of even their most complex actions, conscious of themselves. It is not very long, however, before the human animal begins to set itself off against the remainder of the universe, to discover that it is something different from the chairs, tables, and surrounding people and faces that at first constitute for it only a "blooming, buzzing confusion." A human being performs actions with a feeling of awareness; he is conscious of himself. This consciousness of self (see chapters VII and VIII) becomes more acute as the individual grows older. It has consequences of the gravest character in social, political, and economic life. It is a large factor at once in such different qualities of character as ambition, friendship, humility, and self-sacrifice, and is responsible in large measure for whatever truth there is in the familiarly spoken-of conflict between "the individual and society."