The present article, based on more detailed work by the author (Wegener, 2004) deals with Freud’s Project of a Scientific Psychology. Taking into account the con¬text in which the manuscript was written in 1895, namely, Freud’s correspondence with Wilhelm Fließ, the Project is seen as a letter in the proper sense of the word and, there¬fore, is distinguished from writing intended for publication. Published posthumously and surreptitiously in 1950, it is read as a purloined letter. Lacan’s revisiting of the Project and his interpretation of it as a Schreibspiel, as well as the genesis of Freud’s letter, are stud¬ied. Here, the author focuses more specifically on the precarious position of the neurons in the Project and finally on the function of Fließ as addressee of the Project and as Freud’s Other.