Based on Barbey d'Aurevilly's short story, “Le plus bel amour de Don Juan”, taken from the collection Les diaboliques , the author suggests that the hallucinated sexual theory at the centre of the short story is not a delusional melancholic construction revealing good qualities of mentalization, as was the case, for example, with Haitzmann, as reported by Freud. On the contrary, it is a failure of mentalization that exists in character neuroses, as Michel Fain shows.