ABSTRACT
Our first learning experiments dealt with the acquisition of con cepts of conservation of physical quantities. The cross-sectional studies had already clarified some of the epistemological and struc tural aspects of concepts of conservation in general and of conserva tion of physical quantities in particular. A grasp of these concepts indicates the presence of an underlying system of mental operations that is characterized essentially by two forms of reversibility, inver sion or cancellation on the one hand, and compensation of reciprocal relationships on the other. The particular psychological interest of conservation tasks lies in the fact that they elicit judgments and argu ments expressing these two forms of reversibility.
