After a brief discussion of the concept of statistical, social and functional norm, the authors stress that the latter is related to the constant evolution and hence uncertain nature of scientific knowledge. With regard to mental disease it is underlined that, during the history of psychiatrics, social aspects have often been confused with functional ones, and uncommon statistical aspects have been interpreted as abnormal. Likewise, as it changes over time even the social norm has interfered with functional norms and consequently with the pathological process.