The advent of the information processing approach, allied with the complexity of J. Piaget's theory and his difficulties in accounting for stage transitions through the concept of equilibration, naturally led psychologists to reformulate cognitive development in terms of an increased capacity to process information. Working memory is conceived as the set of schemes that are activated either by mental attention or spontaneous attention following J. A. Pascual-Leone and M. H. Johnson's terms. The intuitions that led to the M-operator model were subsequently extended within the theory of constructive operators. In keeping with the trend towards analysing cognition in terms of information processing during the 1960s, the main intuition of Pascual-Leone was that the successive developmental stages described by Piaget's theory could be ordered along a natural scale of informational complexity of the tasks that children can solve. The emphasis in mnemonic processes introduced by H. A. Simon will be retrieved in all the subsequent information processing accounts of cognitive development.