It is well known that during his studies Freud carefully followed Franz Brentano's lectures and seminars. We often have more difficulty in identifying the traces of this teaching in Freud's work. When he destroyed his youthful archive, Freud was amused in advance by the difficulties facing his “future biographers”. One clue, however, was accidentally preserved, hidden in a letter to Fliess, where Freud mentions a book that had helped him “make a lot of progress”. This was The Function of Judgement by Wilhelm Jerusalem, a philosopher who enjoyed a certain recognition in Vienna at the beginning of the 20^th century. The present article attempts to show how this little-known opuscule was a decisive intermediary between the teaching received from Brentano and psychoanalysis. It was in the wake of Jerusalem's critique in it of Brentano's philosophy that Freud sketched out his Project for a Scientific Psychology, before embarking on the long text of psychoanalysis. It was this theory of judgement that would be further elaborated in “Negation”.