On Saturday, December 2nd 2017, Bernard Lecœur shared with the public present in Toulouse (France), the point of his current reflection on the epistemic value of what Lacan evokes in his seminar regarding a third sex. Thanks to the clinical example of transgender children and after a tour of the Freud's work and Lacan's teaching, he developed the importance of this Lacanian research for the contemporary consideration of the issue of sex. Francis Ratier, regional delegate Midi-Pyrénées de l'ACF presented the conference and chaired the debate.