In what sense may we regard psychoanalysis as a science? This is the question which leads this dissertation. Although its known that Freud, during all his works, has conceived psychoanalysis as a science, epistemological meaning of psychoanalysis remained controversial during last centurys philosophical debate on sciences. This debate has been generally oriented by differenciation between human sciences and nature sciences, which origins are to be sought in the second half of nineteenth century. Confronting positivisms theory of sciences and the new epistemology proposed by Wilhelm Dilthey in his Introduction on spirit sciences, this dissertation aims to demonstrate that Freuds notion of science approaches in some main aspects the epistemology of spirit sciences brought out by Dilthey.