Examining Piaget’s paper Les trois structures fondamentales de la vie psychique : rythme, régulation et groupement of 1942, the paper shows that it represents an important step in Piaget’s thinking, from three standpoints. First, it stresses the functional unity of the development. Then it displays a change of Piaget’s mind toward the problem of affectivity, for Piaget gives up a former project of drawing a parallel between cognitive and affective aspects of development. Finally, the 1942 paper establishes the foundations for the dynamic equilibrium theory, arguing implicitly against the temporal model of development build by the philosopher Jean de la Harpe.