by writing sequenced textbooks and by organizing school classes according to calendar age, schoolmasters invented, as it were, the stages of childhood. Our notions of what a child can learn or ought to learn, and at what ages, were largely derived from the concept of a sequenced curriculum; that is to say, from the concept of the prerequisite…. the mastery of the alphabet and then mastery of all the skills and knowledge that were arranged to follow constituted not merely a curriculum but a definition of child development. By creating a concept of a hierarchy of knowledge and skills, adults invented the structure of child development