Philippe Chaslin proposes a return to a pure clinic that excludes etiological hypotheses because the cosmography of his symptomatic complexes, lacking causal determination, cannot constitute a coherent ensemble. Therefore, he sets up a classification of clinical types, whose logic, following that of B. Russell, encounters multiplicity as the limit of its classification. He is thus lead to found a paradoxical class, that of discordance, which gives rise to the determination of a central disorder, specific to discordant insanity, in which P. Chaslin isolates a clinical type, “discordant verbal insanity”, characterized by psittacism, that is, the cleavage between language and intelligence. Whereas psittacism in the domain of the pathological presents itself as a manifestation of a dysfunction in the act of speech, within the domain of symbolic logic, it belongs to the order of mathematical invention. In his essay on “The psychological mechanism of pure mathematical operations”, P. Chaslin questions the enigma of the objectivation of mathematical entities made by the avant-garde mathematicians of his era. At the crossroads of a question oriented on the one hand by the clinical case approached using the signifying chain, and on the other by the mathematical invention in its littoral function, P. Chaslin circumscribes a real that is manifest within the symbolic in the form of a paradox. In so doing, he has come to the edge of an epistemological reversal.