This article has as its theme the relationship between "power", "reason" and "religion" in O alienista, one of the central texts of Machado de Assis, written in 1881 and initially published in eleven chapters, between October 1881 and March 1882. It is intended to identify and analyze the power relations between the State ("Throne") and the Catholic Church ("Altar"); as well as to highlight the relations between "reason" (science) and its "absence" (madness); without neglecting the Machadian denunciations and criticisms of (...) scientism prevailing in the nineteenth century. To do so, at first, it is presented, even if summarily, the Author and the Tale; in a second moment, shedding lights on its "space", "time" and "action" its plot, necessary assumption for the last moment, where the "Power", "Reason" and "Religion" will be analyzed. This research is justified by the relevance of the author, the place that the alienist occupies in the set of his works, as well as the social relevance of his ironic criticisms. In this analysis, a bibliographic method of critical-analytical reading of the Tale was used, using also the interpretations of some scholars, listed in the References. (shrink)