First published in English in 1969, the book opens with a chapter by Pierre Oléron on intellectual activities. These fall into three groups: inductive activities (the apprehension of laws, relations and concepts), reasoning and problem solving. It describes typical methods and essential results obtained by relevant experiments.

There are two chapters by Jean Piaget and his collaborator Bärbel Inhelder. The first, on mental images, breaks new ground: it describes original experiments carried out by Piaget and associates with children of various ages. Piaget examines the relations between images and motor activity, imitation, drawing and operations. He also classifies images according to their degree of complexity and show why children have inadequate images of some processes. The second chapter is on intellectual operations and Piaget gives a summary of the main findings of a number of his earlier books, on the child's notions of conservation, classification, seriation, number, measurement, time, speed and chance.

In the last chapter, Pierre Gréco discusses learning and intellectual structures. He describes the work of psychologists with rats in mazes and formulating theories of animal learning. Gestalt psychology and various other interpretations are examined and Greco also pays attention to Piaget's view of 'structural learning' based on experience.