This is Volume XX of thirty-two in the Developmental Psychology series. Initially published in 1954, in Piaget's words the study of sensorimotor or practical intelligence in the first two years of development has taught us how the child, at first directly assimilating the external environment to his own activity, later, in order to extend this assimilation, forms an increasing number of schemata which are both more mobile and better able to inter-coordinate. This study looks at the second part of evolution of sensorimotor intelligence, as the description of behavior no longer suffices to account for these new products of intellectual activity; it is the subject's own interpretation of things which we must now try to analyze.