New evidence reveals that Sigmund Freud documented his belief in the central role of sex by his own behavior. A Swiss hotel register shows that he signed in with his wife's sister as "Dr Sigm Freud u frau," the German abbreviation for "Dr. Sigmund Freud and wife."
While an affair with the sister, known as "the Minna matter," as opposed to his wife, the Martha matter, has long been the subject of conjecture, the hotel record seems to confirm that he did enjoy a sexual dalliance with the other lady Bernays.