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The purpose of this study was to show that criteria for assessing logicalness based solely on extensional quantification and its various « traps » are not sufficient for accurately delimiting developing rationality. This theoretical and methodological limitation is clearly illustrated here for the case of categorization. We compared the extensional logic performance (success or failure in Piaget's quantification of inclusion task) and the intensional logic performance of 72 children between the ages of 5 and 8. Three intensional logic tasks were used to test for meaningful implication : Forced- Choice- Transitivity, Entailment, and Partition. The results indicated that already at the extensional logic failure level (preoperational, non-including level), children are « intensional logicians ». The discussion deals with the relationship between rationality and the inhibition of extensional reasoning biases.
