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English summary: The Roman Republic did not emerge fully armed from Montesquieu's brain. In a city where religion and politics were indivisible, the balance of "powers" was envisioned quite differently than it was by the Enlightenment's thinkers. It was not so much a matter of internal division within the sphere of power (potestas) as it was of counterbalancing power through an external authority source (auctoritas). Although it may have been commonplace for an aristocrat to assume the duties of both potestas and of auctoritas, he could not simultaneously act as a senator or priest (an auctoritas duty) and as a magistrate (a potestas duty), since the separation of such functions and their obligatory collaboration constituted the cornerstone of the Republican institutions. The right of the people's magistrates to take auspices - a legacy of the former patrician nobility's prestigious auspicial monopoly - was the Gordian knot in the institutional coupling of the potestas to the auctoritas. Magistrates, whose power was purposely flawed in that it was deprived of any intrinsic legitimacy, had to regularly solicit, under the watchful eye of the Augurs and the Senate, the optimisation of their acts of potestas via the auctoritas of Jupiter. This process of tenuously legitimising the magistrates' power was thus an integral part of the Roman aristocracy's self-control mechanisms. French description: La Republique romaine n'est pas sortie tout armee du crane de Montesquieu. Dans une cite ou religion et politique etaient indissociables, l'equilibre des pouvoirs etait envisage tout autrement que chez les penseurs des Lumieres: il s'agissait moins d'y parvenir par une division interne a la sphere du pouvoir (potestas) que de contrebalancer le pouvoir par une source d'autorite exterieure (auctoritas). S'il etait courant qu'un aristocrate cumule la fonction de potestas et la fonction d'auctoritas, il ne pouvait agir simultaneme