The goal of this paper is to show how the study of history and philosophy of mathematics can shed light on becoming conscious of the existence of cognitive conflicts in the pedagogical activity. The proof of the Pythagorean theorem is taken as a particular illustration of that situation. When a mathematical concept is simplified for the sake of didactical aims, some methodological confusions emerge and these may become unsurmountable barriers for the student. Teachers, as well as students, are not just unaware of the origins and causes of a conflict of such a nature in the learning of mathematics but, in some occasions, they do not realize the conflict itself.
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