Your impression now will be that we have reduced want of satisfaction to a factor of negligible proportions by the recognition of so many means of enduring it. But no; this is not so: it retains its pathogenic power. The means of dealing with it are not always sufficient. The measure of unsatisfied Libido that the average human being can take upon himself is limited. The plasticity and free mobility of the Libido is not by any means retained to the full in all of us; and sublimation can never discharge more than a certain proportion of Libido, apart from the fact that many people possess the capacity for sublimation only in a slight degree. The most important of these limitations is clearly that referring to the mobility of the Libido, since it confines the individual to the attaining of aims and objects which are very few in number. Just remember that incomplete development of the Libido leaves behind it very extensive (and sometimes also numerous) Libido-fixations upon earlier phases of organization and types of object-choice, mostly incapable of satisfaction in reality; you will then recognize fixation of Libido as the second powerful factor working together with privation in the causation of illness. We may condense this schematically and say that Libido-fixation represents the internal, predisposing factor, while privation represents the external, accidental factor, in the ætiology of the neuroses. I will take this opportunity to warn you against taking sides in a quite superfluous dispute. It is a popular habit in scientific matters to seize upon one side of the truth and set it up as the whole truth, and then in favour of that element of truth to dispute all the rest which is equally true. More than one faction has already split off in this way from the psycho-analytic movement; one of them recognizes only the egoistic impulses and denies the sexual; another perceives only the influence of real tasks in life but overlooks that of the individual’s past life, and so on. Now here is occasion for another of these antitheses and moot-points: Are the neuroses exogenous or endogenous diseases—the inevitable result of a certain type of constitution or the product of certain injurious (traumatic) events in the person’s life? In particular, are they brought about by the fixation of Libido and the rest of the sexual constitution, or by the pressure of privation? This dilemma seems to me about as sensible as another I could point to: Is the child created by the father’s act of generation or by the conception in the mother? You will properly reply: Both conditions are alike indispensable. The conditions underlying the neuroses are very similar, if not exactly the same. From the point of view of causation, cases of neurotic illness fall into a _series_, within which the two factors—sexual constitution and events experienced, or, if you wish, fixation of Libido and privation—are represented in such a way that