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Psychopathological phenomena can be explained in two ways: by their form “the what and the how” or by their function “the why.” These two positions still give different results, probably because their conceptual tools are difficult to reconcile. In 1925, based on a phenomenon that would today be related to intellectual development disorder, Wallon offered us a brilliant synthesis of these two positions as part of his thesis To do this, he adopted an integrative and developmental position, which will be illustrated through a clinic of stereotypies. Developmental psychopathology was already in the making.