In short, the reader will be presented not only with a number of experimental facts but also with several theoretical notions, the validity of which the future alone can decide. For the rest, there is one aspect of these studies that has greatly encouraged us in our work: the surprising discovery, in a sphere apparently remote from that of cognitive operations, of a precise succession of operational stages, whose existence we ourselves might have begun to doubt had we listened to all those who do not, or rather do not yet, believe in the validity of the operational approach.
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