The basic aim of the study was to conduct a conceptual analysis of Piaget's hypothesis on horizontal displacement (HD hypothesis). This part of theory has not been sufficiently elaborated by Piaget himself and has been frequently criticized as the weakest link in the Piaget's theory. Some basic concepts of the Piaget's theory (global stage of cognitive development, cognitive structure and action patterns) and their interrelations have been analyzed. However, these concepts failed to enable the

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... iaget's theory to explain the finding that children with certain cognitive structure fail to solve all tasks for which the given cognitive structure is needed. In other words, there is a displacement in age when children become able to solve the tasks applying the same logical forms, contradicting the prediction derived from the Piaget's basic concepts. In order to correct the imbalance of theory and empirical findings, Piaget sketches the basic idea of HD hypothesis. The basic objections to HD hypothesis include its "brief description" of obtained information suggesting that it is only an ad hoc hypothesis. Both objections have been analyzed and proven unfounded, suggesting thus, that HD hypothesis has to pass an empirical test. In order to enable the empirical evaluation of HD hypothesis the second part of the study contains a derived analysis and precise elaboration of HD hypothesis. Thus, it has been established that it can be treated as a bifactorial model where the cognitive structure defines the cognitive potential of children (competence), while the problem itself promotes or hinders the application of a certain cognitive structure on the given problem (performance).