*The Riddle of the Sphinx.*--It is not theoretical but practical interests which start the work of the investigation activity in the child. The threat to the conditions of his existence through the actual or expected arrival of a new child, the fear of the loss in care and love which is connected with this event, cause the child to become thoughtful and sagacious. Corresponding with the history of this awakening, the first problem with which it occupies itself is not the question as to the difference between the sexes, but the riddle: from where do children come? In a distorted form, which can easily be unraveled, this is the same riddle which was given by the Theban Sphinx. The fact of the two sexes is usually first accepted by the child without struggle and hesitation. It is quite natural for the male child to presuppose in all persons it knows a genital like his own, and to find it impossible to harmonize the lack of it with his conception of others. *The Castration Complex.*--This conviction is energetically adhered to by the boy and tenaciously defended against the contradictions which soon result, and are only given up after severe internal struggles (castration complex). The substitutive formations of this lost penis of the woman play a great part in the formation of many perversions. The assumption of the same (male) genital in all persons is the first of the remarkable and consequential infantile sexual theories. It is of little help to the child when biological science agrees with his preconceptions and recognizes the feminine clitoris as the real substitute for the penis. The little girl does not react with similar refusals when she sees the differently formed genital of the boy. She is immediately prepared to recognize it, and soon becomes envious of the penis; this envy reaches its highest point in the consequentially important wish that she also should be a boy. *Birth Theories.*--Many people can remember distinctly how intensely they interested themselves, in the prepubescent period, in the question where children came from. The anatomical solutions at that time read very differently; the children come out of the breast or are cut out of the body, or the navel opens itself to let them out. Outside of analysis one only seldom remembers the investigation corresponding to the early childhood years; it had long merged into repression but its results were thoroughly uniform. One gets children by eating something special (as in the fairy tale) and they are born through the bowel like a passage. These infantile theories recall the structures in the animal kingdom, especially do they recall the cloaca of the types which stand lower than the mammals.