This paper is a reading of Fustel de Coulanges ’1866 Ancient city : a study on the religion, laws and institutions of Greece and Rome through Ernest Renan. The idea is to give the way Fustel de Coulanges locates the Greeks and the Romans within an Indo-European heritage a context, by highlighting three structural points he has in common with Renan : the inheritance of Burnouf’s « Aryanist » and spiritualist approach ; the articulation of an interior and an exterior history of the mind ; the paradoxical temporality of a history which expands between an initial emergence where everything is given and the continuity of a development where renewals multiply. In this context, Fustel’s originality stands out in the fact that the documents of institutional history which give him his sources for the life of the mind suppose the recognition of a diversity and malleability in Indo-European beliefs which makes them impossible to oppose to that of other peoples as buoyantly as Renan would have it.