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This article presents Lucien Cotard’s medical thesis (1908) devoted to the study of the symptom called psittacism (parrot language), designating speech without thought. This young physician’s work belonged to the field of the symptomatology of madness and more generally the relationship between words and ideas. We highlight the contribution of the alienist Jules Séglas and the philosopher Ludovic Dugas to this thesis. Cotard’s medical treatment of a subject that was originally part of philosophical psychology is examined. We explain how the author classified the different varieties of the symptom observed in his clinical practice and how he described the psychological processes responsible for this language disorder. Finally, we explore the fate of this notion in twentieth-century psychiatry by emphasizing its links with the concept of verbal discordance developed by Philippe Chaslin.