Outlined are some specificities of obsessive-neurotic and paranoid patients and their modalities of thinking viewed from the standpoint of formal logical operations and socio-ideological stances. The obsessive doubter seems to be trapped in the hypothetical premise; thus his thought processes lead to a kind of circular perpetual motion, unresolved by the certainty of a conclusion formulated as a categorical assertion. At best such patients, mobilizing the compulsive isolation mechanism, arrive at a disjunctive classification of reality and a rigid categorization of their social field, its incompatibilities and prohibitions. By contrast, paranoids overcome their basic distrust by the defense of the absolute certainty of their delusional idea, which serves as a kind of anti-Archimedean fixed point