The title of my presentation reformulates the central postulate of Piaget’s theory: that a new structure can only be built on the basis of an existing structure related to the same kind of problem. I will first explain how Piaget used this postulate as the foundation for his work on the birth of intelligence. Then I will illustrate this process by means of the development of prehension behavior. I will examine several oppositions between practical intelligence and conceptual intelligence, and reconsider the use of the terms “practical” and “conceptual” to differentiate between systems of knowledge at different levels of development. My research on the construction of simple tools by children aged 4 to 9 years old will illustrate in more detail the process of developing new skills on the basis of pre-existing skills (that are simultaneously practical and conceptual). I will conclude my presentation by discussing some problems in the history of scientific and technical knowledge that are comparable to those discussed in relation to practical and conceptual skills in child development.