Freud's 1897 treatise on infantile cerebral paralysis, based on clinical observation, and his Interpretation of Dreams, based on psychological self-observation, are compared in order to gain insight into the development of Freud's theory. It is apparent that the concept of a more highly developed system suppressing a more primitive one (and the possibility of an eruption of the suppressed system) played an important role even in Freud's prepsychological writings, as did the concept of muscular and ideational work.