At one end of the series are the extreme cases, of which you can say with full conviction: These persons would have become ill because of the peculiar development of their libido, no matter what they might have experienced, no matter how gently life might have treated them. At the other end are cases which would call forth the reversed judgment, that the patients would undoubtedly have escaped illness if life had not thrust certain conditions upon them. But in the intermediate cases of the series, predisposing sexual constitution and subversive demands of life combine. Their sexual constitution would not have given rise to neurosis if the victims had not had such experiences, and their experiences would not have acted upon them traumatically if the conditions of the libido had been otherwise. Within this series I may grant a certain preponderance to the weight carried by the predisposing factors, but this admission, too, depends upon the boundaries within which you wish to delimit nervousness. Allow me to suggest that you call such series _complementary series_. We shall have occasion to establish other series of this sort. The tenacity with which the libido clings to certain tendencies and objects, the so-called _adhesiveness_ of the libido, appears to us as an independent factor, individually variable, the determining conditions of which are completely unknown to us, but the importance of which for the etiology of the neuroses we can no longer underestimate. At the same time we must not overestimate the closeness of this interrelation. A similar adhesiveness of the libido occurs--for unknown reasons--in normal persons under various conditions, and is a determining factor in the perverse, who are in a certain sense the opposite of nervous. Before the period of psychoanalysis, it was known (Binet) that the anamnesia of the perverse is often traced back to an early impression--an abnormality in the tendency of the instinct or its choice of object--and it is to this that the libido of the individual has clung for life. Frequently it is hard to say how such an impression becomes capable of attracting the libido so intensively. I shall give you a case of this kind which I observed myself. A man, to whom the genital and all other sex stimuli of woman now mean nothing, who in fact can only be thrown into an irresistible sexual excitation by the sight of a shoe on a foot of a certain form, is able to recall an experience he had in his sixth year, which proved decisive for the fixation of his libido. One day he sat on a stool beside his governess, who was to give him an English lesson. She was an old, shriveled, unbeautiful girl with washed-out blue eyes and a pug nose, who on this day, because of some injury, had put a velvet slipper on her foot and stretched it out on a footstool; the leg itself she had most decorously covered. After a diffident attempt at normal sexual activity, undertaken during puberty, such a thin sinewy foot as