ABSTRACT
We know that the idea of horizontality and verticality (for example, the ability to predict the inclination of the surface of a liquid in a jar about to be tilted in specified ways) is not acquired by children before the age of nine to ten years. By contrast, the perception of horizontality, though, of course, modified at the age of nine to ten years by the growing influence of operational procedures,2 provides much younger subjects with a rough but adequate idea of the relation between the horizontal and their own line of vision and bodily position.
