It will be seen, then, that my mistake was of the same kind as would be made by someone who believed that the legendary story of the early kings of Rome (as told by Livy) was historical truth instead of what it is in fact—a reaction against the memory of times and circumstances that were insignificant and occasionally, perhaps, inglorious. When the mistake had been cleared up, the path to the study of the sexual life of children lay open. It thus became possible to apply psychoanalysis to another field of science and to use its data as a means of discovering a new piece of biological knowledge. The sexual function, as I found, is in existence from the very beginning of the individual’s life, though at first it is assimilated to the other vital functions and does not become independent of them until later; it has to pass through a long and complicated process of development before it becomes what we are familiar with as the normal sexual life of the adult. It begins by manifesting itself in the activity of a whole number of _component instincts_. These are dependent upon _erotogenic zones_ in the body; some of them make their appearance in pairs of opposite impulses (such as sadism and masochism or the impulses to look and to be looked at); they operate independently of one another in their search for pleasure, and they find their object for the most part in the subject’s own body. Thus, to begin with, they are non-centralized and predominantly _auto-erotic_. Later they begin to be co-ordinated; a first stage of organization is reached under the dominance of the _oral_ components, an _anal-sadistic_ stage follows, and it is only after the third stage has at last been reached that the primacy of the _genitals_ is established and that the sexual function begins to serve the ends of reproduction. In the course of this process of development a number of elements of the various component instincts turn out to be unserviceable for this last end and are therefore left on one side or turned to other uses, while others are diverted from their aims and carried over into the genital organization. I gave the name of _libido_ to the energy of the sexual instincts and to that form of energy alone. I was next driven to suppose that the libido does not always pass through its prescribed course of development smoothly. As a result either of the excessive strength of certain of the components or of experiences involving premature gratification, _fixations_ of the libido may occur at various points in the course of its development. If subsequently a repression takes place, the libido flows back to these points (a process described as _regression_), and it is from them that the energy breaks through in the form of a symptom. Later on it further became clear that the localization of the point of fixation is what determines the _choice of neurosis_, that is, the form in which the subsequent illness makes its appearance.