Based on the hypothesis that the construction of number is closely related to the development of logic, this book records a series of experiments investigating classes, relations, and numbers as cognitive domains. The author finds that number is organized, stage after stage, in close connection with the gradual elaboration of systems of inclusions (hierarchy of logical classes) and systems of asymmetrical relations (qualitative seriations), the sequence of numbers thus results from an operational synthesis of classification and seriation. Logical and arithmetical operations therefore constitute a single system that is psychologically natural, the second resulting from generalization and fusion of the first, under the two complementary headings of inclusions of classes and seriation of relations. When the child applies these operations to sets that are defined by the qualities of their elements, he is compelled to consider separately classes and asymmetrical relations hence, the dualism of logic of classes and logic of asymmetric relations relations. But when the same system is applied to sets irrespective of their qualities, the fusion of inclusion and seriation of the elements into a single operation totality occurs, and this totality constitutes the sequence of whole numbers, which are indissociably cordinal and ordinal. (BR)