International audience It first began with an attention to the way arab thinkers say words to each others. At the very beginning of the Xth century, Abū Ḥātim ar-Rāzī accused Abū Bakr to be an unbeliever. Two centuries later, Ghazālī speaks in terms of stupidity of the philosophers. Something happened in between. The arab thinkers pillory their predecessors and contemporaries for their stupidity. But we cannot understand the stupidity told about the arab thinkers if we don't understand the stupidity they spoke about, the precise and effective concept they built. This very concept reveals the central caesura of intellectual life, the caesura which separates reason and unreason and gives its norms to the thought. At this time, unreason is not the talkative madness but rather stupidity, which is a half-truth, assured by its logic, that leads to wander away from evident truth and fundamental morality. Stupidity characterize a half-truth which knowledge make thinkers renounce to obviousness and throw them into damnation. Stupidity is the paradox of moving away from truth in struggle to reach it. A synthesis of this concept can be read in the Revival of Religious Sciences, but philological analysis shows that Ghazālī inherits it ungratefully from a philosophic tradition which roots in the 10thcentury, especially in Fārābī's and Miskawayh's textes and can be still be tracked until the end of the 12th century in the book wich made this tradition popular and eternal, I mean the Ibn al-Jawzī's chronicles of stupids and idiots. The precision of the concept, the deep consciousness of its definition in great arabs' thought and the existence of an important literature on the character of idiots allows to found a history ofunreason at the age of reason in islam.