A study of negative polarity idioms –NPI– (e.g. not lift a finger, not say a word) within the theoretical framework of stereotypical theory (Kleiber, Anscombre) is proposed in our paper. From our point of view, generic phrases underlying sentences with NPIs play a crucial role in the presence of a negation. As a matter of fact, a sentence like Peter didn’t lift a finger for his sister doesn’t actually deny *Peter lifted a finger for his sister but rather a sort of general rule related to stereotypical contents: “people help their close relations”. Negation in NPIs is then a trace, a sign of this opposition between a general belief and a particular case which seems to be a counter-example. This perspective shows several common points between phrases including NPIs and phrases including positive polarity items.