First published in 1997. This is Volume II of selected works of Jean Piaget which looks at his thinking on children and how their interaction and perception of number, touching on the areas of development in thinking on conversation of quantities and invariance of wholes; cardinal and ordinal one-one correspondence; as well as additive and multicatitive compositions. These explorations look into tracing the development of the operations which give rise to number and continuous quantities, to space, time, speed, etc., operations which, in these essential fields, lead from intuitive and egocentric pre-logic to rational co-ordination that is both deductive and inductive.