Smedslund, J. The acquisition of conservation of substance and weight in children. I. Introduction.Scand. J. Psychol., 1961,2, 11–20.—This is the first in a series of articles concerning the problem of how children acquire the concepts of conservation of substance and weight. The four alternative interpretations of nativism, maturation theory, learning theory and equilibration theory are discussed. It is concluded that nativism and maturation theory do not account for the known facts. An important issue is whether the concepts are established by external reinforcement (learning theory) or by a process of inner equilibration, as maintained by Pia